When you ask (great question) what aspect of Christianity has been most unbeleived, historically, let's try to take it historically: Was it Ghandi who said: +/- "Christianity is a great idea, too bad so few have ever tried it?"
The one word answer is something called The Incarnation, which is core beleif to most, but not all, faiths that call themselves Christian.
The Incarnation means God becoming IN-carn-(flesh)-ated, meaning, becoming human. This is what is celebrated at Christmas, and it is the (tough) beleif behind the doctrine of the Trinity, Three Persons, One God, core to all orthodox Christianity, even "Bible only" churches. When Jesus' followers were confronted with their own immediate beleif that Jesus is God even though he was plainly human, they faced this incredible Incarnation. Except Mary, who had some of the best proof first, they had a really hard time, if you read the New Testament, beleiving the Incarnation.
However, without beleif in the Incarnation, there would never be Christianity.
To this day, the hows and whys of Incarnation are THE ongoing difficult beleif, inside and outside of Christian faith: Why would God become so low as to become a human? (much less die on a cross) Wasn't there an easier way to get the job done? Is there still one God if this happens? What does that mean about what we should accept about God being all powerful, unchangeable, the Creator, etc? The Incarnation challenges all that people are supposed to beleive about God. It also says important things about the value of being human and the value of the material natural world and suggests things about how to face the bad things in life with failure, evil, and suffering.
Beleiving in the Incarnation is beleiving in God's move, not ours: it is not about what relationship we WANT to have with a god: for god to be the boss, take all initiative, telling us what to do, to fix things, and make us happy, rich, and powerful. Instead, with the Incarnation, the ideas is that when God had a say, He would rather go as low as humanly possible, and even lower perhaps, and be not rich, not powerful, not pain-free, not accepted, just to make the point that the non-user relationship between Him and us was most important to Him.
Being Catholic, this is a rich, day to day, work of faith to "get" the Incarnation. I can embrace and love the Incarnation overall as a message of love to me and my children from God, and it colors my developing understanding of God's personality and character in a very good way, but I still do not "get " the applications of the Incarnation as I face life. The difficulty of this beleif is the "work" of a Christian.
So I can attest that those Christians who profess the Incarnation, historically, as intsitutions and in their daily faith life, find this one Incarnation aspect of Christianity the hardest to beleive.
As a Catholic, I can tell you that the difficulty beleiving the Incarnation can be seen as the reason for there being so many Christian denominations. Catholics, including the very early Christians, as described in an early historical document called the DIDACHE, which you can look up, beleived the Incarnation was extended through to the Real Presence of Jesus in the consecrated sacrament of the Eucharist. Because of the ease of abuse of material things, the concrete nature of the sacraments faced challenges, and not all was won. A core division between Christians can be seen as differences in beleif in the Real Presence of Jesus in the transformed bread and wine from Mass. If only a symbol, if The real Incarnate Presence is not real, it therefore is ok to skip, to sideline, or to not make any further distinction between when you beleive it or not.
One of the critical aspects of the difficulty of beleif in the Real Presence of Jesus in the Eucharist, as well as the overall beleif in the Incarnation, is that it is so very easy NOT to beleive. It is so very easy NOT to beleive the Incarnation, that it is your free will whether to beleive it or not, especially as a religion, faith or doctrine. Thank God it really isnt so hard to begin to beleive in the Incarnation because it marks the start of a relationship with the Person of Jesus. The difficulty is its not static, when beleif in the Incarnation begins to grow, when you have to face Gods presence in something, like any relationship, if it doesnt grow it dies.
Other denominations may say the division in Christianity is instead over strict faithfulness to the Bible or not, but from my view, the more you study the Bible, its origins, and how it really describes Christ's life and words, along with accounting for what the early church beleived before the Bible itself was codified or canonized hundereds of years after Christ, that division over the Bible argument gets tipsy. The division in the Church is a division over beleif in the Incarnation in the Real Presence of God in the Blessed Sacrament, which is...not easy to beleive.
Other religions, specifically Islam, or other reigions such as Mormons or Jehovah's Witnesses, have defined themselves specifically by taking a definitive strong position to stand and refute ANY sense of the Incarnation of God. Signal is the rejection of the idea of Three persons, one god, the Holy Trinity.
For Islam, which perhaps better understands than many Christians what Christianity purports in the Incarnation, the Incarnation is the core blasphemy that must be stood up against or else discredit God. They honestly beleive that the suggestion of the Incarnation is offensive to God, and teh suggestion of the Incarnation tries to buck the authority of what Islam sees as the real master servant relationship between God and humans.
But back to Ghandhi's quote: too bad Christianity has never been tried.
The pricily bought message of example acted out by God to not grasp power (the Romans were not overthrown in Jesus' day, rather Romans and Jews powerplayed to crucify Christ),and to not place political or social inclusion and praise above all else, recalling the story of the foreign woman, bold to walk in among Jews and ask a favor of healing for her child. It was obvious, in the context, she was not going to get "in" with the social and religious establishment present in the room. By Jesus' response of giving the healing, (amid the dog and scraps banter?) it was confirmed between the woman and Jesus, the shared value and beleif that it was better to be "in" with God than to be "in" by those worldly standards. This is a "had to be there" kind of thing. It would not have carried had it not been a face to face with God, and the message was primarily between Jesus and the woman. The Incarnation is all about a "you had to be there" kind of thing: a relationship one to one. It did count peripherally that it became broadly known this foreign woman recieved her healed child request, amid the stating of the obvious of her real world status entering that room to get to Jesus. I wish I had the inclusion she had with Jesus. Point taken.
THe critical aspect of the Incarnation is stressed in I John 4:2. If you check it, be sure to go on through the antichrist bit, and dont miss I John 4:5. Basically Saint Paul says that you can test if a message is really Christian by paying attention to one litmus point: Whether its said; did Jesus Christ come in THE FLESH (the Incarnation), or not. The default, easier to beleive, way God would appear to Man is NOT in the flesh Himself, but rather like a ghost, a vision, sending a messenger,a tablet etc, not actually IN THE FLESH. Paul goes on to say whatever anyone else is offering who says otherwise (while rejecting the Incarnation) is trying to offer some other worldly (ie man-made, man-conceptualized, man-manipulated) good.
The Incarnation, that is so hard to beleive, says that a person's relationship with God is not only more important but truly better than anything in the world you would ever want: being "right" being "in" politically or socially now, being the God of your own new planet in heaven, 50 virgins coming to meet you, or whatever. Listen to nonbeleivers to tune in and most clearly see through the many times in many religions when the candy wrapper is being crinkled to bribe and entice a person under the misused power of religion.
Basically, THe Incarnation posits that the person's individual relationship with God is better than anything that can be cello-wrapped and sold off the shelf as reward for "being good." How un-useful. What's more, the gift of the Incarnation, when embraced, which is not easy, can not diminish but rather can empower and value a person in the middle of pain, injustice, servility, oppression, failure, invalidity, poverty or weakness. Being close friends with God is worth going through any fire, loss, suffering, loss, or injustice, not so much because God asks that of us, but because God embraced it Himself Personally:
Pretty much THE INCARNATION is God saying "hey, I'm the One who wants to be with you." (I love that Mr Big song for as long as it carries parallel with this meaning.)
On the other hand, one disjoint failing to really beleive in the Incarnation when you think you are "on it", is when beleivers begin to make a big deal of the sacrifices a person can make. Kind of a pain competion with a human winner. The Incarnation just posits that no matter how low a person goes, God is ahead of you, and therefore, bigger. There is no low place (physically, morally, intellectually) a person can go in this life to be away from GOd. I heard it said once that in a Marian apparition in Medjugorje (whether that's beleivable to a reader or not): Mary said she wished she could be the worst sinner and still never have offended GOd. This would fit what I do beleive in the Incarnation because its fits my beleif that Mary, as the prototype Christian, loves, loves, LOVES being saved by God. She comparatively had it good compared to other sacrifices made by others in history, why would she be also considered by Catholics at the first of the martyrs?
Which brings another point of how the Incarnation is the hardest to beleive aspect of Christianity, besides how both Christians and nonchristians have great difficulty really beleiving it. From my tradition, we hear that even some angels could not accept it:
It is shared within my tradition a scenario that 1/3 of all the angels fell because THEY would not accept the plan of Incarnation. These angels included their leader, the angel of light, Lucifer. THe words of rejection attributed Satan and his hosts were : "I will not serve." Its as though God did a housecleaning in heaven of all his servants there who did not have humility to match God's, because in fact, God is God and if you beleive in a God, He is the one above all. How can it be that the all powerful God is in fact "meek and humble in spirit" as scripture says? He cant have pride hanging around. Cheif housekeeping tool? THe Incarnation. Love it or leave it. FYI this fits Catholic beleif that Mary is the created human first redeemed of God, the prototype Christian signaling what God will do for us and his Creation. Catholics beleive Mary is made queen of all angels. The Angels who couldnt handle being put under the rule of humans, left of their own accord. THe ones left are the obedient and humble angels that are totally happy at all good coming to people and the rest of God's creation.
You very much can embrace the meaning of the Incarnation in a happy good pain free life: all you have to do is focus on the Giver, not the gift. This also leads to responsibility (not to mention love and enjoyment) toward creation and nature: The earth is not all bad: We are to love life and nurture what is good. If we beleive humans are the ones made in the image of God to understand natural systems, lifecyles etc and take creative initiative to cherish the nature that God did not recoil from.
At the last supper, on top of all the body language of Christ's life on earth, The Incarnation, Jesus makes the point of taking off his outer clothes and taking the lowest slave's job of washing the disciples feet. Just in case the Crucifixion was not apparent to be deliberate, it is Crystal Clear that a basic message of the Incarnation of Jesus is to serve rather than to be served. Jesus said: If I have done this to you, you must do this for others. If a Christian, much less a nonChristian who sees so much of the bad Historical examples of Christianity, can really beleive the difficult to beleive Incarnation, how can one possibly beleive the Incarnation and put him or herself above God by NOT serving others? By preferring instead power, acceptance, dominance, etc?
How difficult is the Incarnation to beleive? Really, really hard. Proof? What's our Christian track record? (By the way, deep in their book on Our Lady of Guadalupe by AndersonChavez there's a great discussion on this)
However, when the Incarnation is beleived, its nature will not be to "blow a trumpet ahead" of one's works: Some rare big examples of beleif in the Incarnation would be Mother Teresa, going out farther to seek out and help the most miserable, and not asking them to change their religion. Many missions of many religions do go out to serve, and without the goal to change a religion. I'd love anyone to become Catholic, but that can'and my personal motivation may be to work out my embrae of the Incarnation, but changing anothers beleif can not be my mrice for service, or else its a price, and then not service. Read or listen on tape to Anderson/Chavez new book on Our Lady of Guadalupe. Learn about Brother Charles Foucald and the White Fathers in North Africa. The Three Cups of Tea author, Mortgenstern. (sorry for the spelling. He is a better person than his work is good; what a sweet man.) I feels odds are good the servanthood and humility could be found somewhere amid the Christains and Muslims building democracy together in the middle east now, specifically Egypt, God bless and help them.
THe White Fathers share a quote (+-) Jesus has taken the lowest place, and it wont be taken away from Him. How often have you heard tales of Christians fighting to take the lowest place? To serve rather than be served? IF historically we had that convincing track record, as the Incarnation calls us to, I wonder would there be Islam at all. As it is, day to day, many Muslims are picking up OUR slack as Christians. If you want to meet a humble person, make friends with a true Muslim. You may find it very refreshing.
The point is, that while Christians historically have made such points to assert power and dominance, who's to say some others should prefer to come along and assert the power and dominance of God? Whos to say thats not the better thing to do, given the context?
This impression very well may not be true, the evil abuses of religion will always get bigger press, and the power grabbers almost always make the biggest splash and drama. We hear the most of the evils done in the name of Christianity and Islam. And Judaism, too, btw. Any organized religion.
When I do not have close to me heroic examples or opportunities to beleive in the difficult aspect of the Incarnation, I have only to face deeply the challenges and failures staring me down in my own life. There is no trouble so deep and dark that facing it, while beleiving in the Incarnation, will not bring me closer to a Person. This is a Person who I have had a glimpse just big enough, to know proves in real action, is someone I want to know much better: GOd. I may only learn a crumb more, but a little knowledge of a wonderful person is what it takes to feel that knowing that person better is worth a whole lot, even if it takes time and other things to get there.
By other things, I could mean heroically facing sacrifices, I guess as one way. Better, though, is that when I as a Catholic, can beleive, or perhaps I should say, have the opportunity to face the difficult challenge of beleiving the Incarnation by spending time with the sacraments, specifically the Eucharist, I have good help. When I face this difficulty to beleive the Incarnation at the same time as I am facing the challenge of sickness or grief or whatever struggle, there are good results for me because the answer is that there exists something worth far more than anything else problem-solving-wise that can be offered. I can gain a deeper relationship with God as an individual who as God, values a personal relationship with His created ones above all else.
Having had God prove this in His Person, by the Incarnation, my work is to move towards shuffling around all other values I might have over God. I can have many of those values, but they must be Under God. It is work, and difficult, but I think it's worth trying.
Best to you, excellent question-asker.
What is th most unbeleivable aspect of Christianity?