Why did I stop believing in Christianity?
1. Jesus was not a Christian! He claimed to be the Jewish Messiah and denied that he intended to altar the Jewish law.
2. I cannot accept that we are born sinners. We are born with the capacity to learn to be good, just as we are born able to learn a language: we just need to be taught and to practice.
3. I cannot accept the doctrine of atonement. If I have committed a crime, then I need to be punished: for the punishment to be visited on some-one else, even if they are willing, makes no sense.
4. I see no evidence for the resurrection. No-one saw it, only an empty tomb. The stories of people seeing the physical body of the risen Jesus are all late. Paul was right in saying that the resurrection was important, so why wasn't it done publicly?
5. Jesus made prophecies that did not come true, like saying that his hearers would live to see the end of the world.
6. I cannot accept his teaching. For example, belief that the faults of one life-time justify eternal punishment seems the hight of immorality; even Islam teaches that hell is not eternal.
7. What of all the people who lived before Jesus? If the "incarnation" made the world a better place, why didn't it happen earlier? If the people who lived earlier were not the worse for it, why did it happen at all?
8. If the creator wanted to communicate to mankind, why do it through one obscure figure in an obscure corner of the world? If course the Baha'i will say he did it many times, but they quote people for whom there is no evidence (Moses) or whose teaching is even worse than that of Jesus (Muhammad).
I could go on, but you get the general idea.