If you feel like sharing the reasons, that would be interesting. There are different types of reasons for doubting and disbelieving Christianity and they require different types of rebuttals - in some cases there is no conceivable rebuttal. Furthermore, there are different types of Christianity and a reason that kills one type of Christianity may only slightly wound another type of Christianity.
Thanks for offering to hear my reasons. I don't have these arranged in any specific order, just what comes to mind. I will try to keep them short, but I can expand on any of them if needed. These are from years of careful study, not knee-jerk reactions.
- The supreme deity of the Bible is an artificial amalgam of El and Yahweh, originally two unrelated deities. (Some parts of the Bible maintain their original distinction!) The monotheism of the Bible is the result of a slow historical process as cultures met and borrowed from each other.
- The Bible presents a universe only a few thousand years old, and a literal pair of first humans, and Paul's soteriology requires the literal truth of these things. The evidence we have shows the universe is billions of years old, the human species is the result of millions of years of evolution, and there were no first humans.
- The Bible presents a universal flood as historical fact, taking place a few thousand years ago, and Jesus' eschatology requires these the literal truth of this event. There is no evidence the global flood happened, and the story in the Bible is physically impossible.
- The Exodus and the conquest of Canaan are the foundation for Israel's history. There is no evidence the Exodus or conquest happened. There is evidence the Israelites were Canaanites. No Exodus means no basis for the claim of Israel's covenant with God.
- The law contradicts itself on a variety of topics. Several parts closely resemble law codes from throughout the region. Israel's law did not come from God, it's an amalgam of several local law codes and added to over time.
- There is evidence a few of the Judges evolved out of pagan gods, demigods, and heroes. The period of the Judges is at least a partial fabrication.
- David's rise from shepherd to king is almost entirely legendary, created from two or three different versions of the same story. The divine ordination of his rule is an invention.
- The kingdom of Israel was never as large as the Bible says it was during Solomon's rule.
- Many of the events from the period of the divided monarchy are not historically attested by Israel or Judah's contemporaries. For some events, such as the death of thousands upon thousands of Assyrians in a failed invasion of Jerusalem, this is extremely implausible.
- The prophets in the Bible, including Jesus, make dozens of unconditional predictions which were never fulfilled.
- The two stories of the virgin birth contradict each other in most of their details, including when it happened. Matthew places Jesus' birth before 4 BC, while Luke places Jesus' birth about AD 6. Matthew borrows from legends about Moses' birth, while Luke borrows ideas from Judges and 1 Samuel. The stories were invented to put Jesus' birth in Bethlehem, since he is otherwise shown belong to Nazareth.
- Some of Jesus' teachings and miracles require a post-70 context to make sense. This means some things in the gospels are much later inventions retroactively attributed to Jesus.
- John's story of Jesus raising Lazarus from the dead is based on Luke's parable of Lazarus and the rich man. An allegory Jesus told in one gospel is reshaped into literal history in a later gospel. There are a handful of other examples like this which suggest John's literary dependence on Luke.
- The crucifixion stories in all four gospels contradict on several minor details, except for John's major contradiction placing Jesus' death a full day before the other three.
- The crucifixion story is full of historically implausible information whose sole purpose is to vilify the Jews and withhold guilt from Rome, despite Jesus being executed by the Romans.
- The most substantial contradiction of all is the nature of Jesus' resurrection appearances. All four gospels show the disciples were still in Jerusalem when the empty tomb was discovered, but none of them agree on what happened next. Mark has no resurrection appearance. (Verses 9-20 are a later addition from someone who had read at least Luke and Matthew.) Luke says Jesus first appeared alive to the disciples in a house in Jerusalem on the same day. (John does the same, but his order of events is different.) Matthew has the first appearance on a mountain in Galilee, requiring the disciples to travel ninety miles from Jerusalem (or farther if they went around Samaria, which was common), which means the disciples could not have seen the risen Jesus for the first time until about a week after the empty tomb was found. This major difference of where and when Jesus first appeared to the disciples is an insurmountable contradiction.
- Luke ends with Jesus' ascension the same day the empty tomb was found, shortly after his first appearance to the disciples. Acts begins by backtracking and adding a forty-day period between Jesus' first appearance and his ascension. These not only contradict each other, but they add another contradiction with Matthew and John. Luke and Acts shows the disciples remained in Jerusalem from the day of the empty tomb all the way through Pentecost. This cannot be reconciled with Matthew's story of the disciples going to Galilee to find Jesus alive on the mountain, nor with John's story of the disciples returning to their fishing life in Galilee.
- The Bible contradicts itself on the ethics required by God and Jesus.
- The Bible contradicts itself on the nature and duration of afterlife punishment.
Sorry if this is too much all at once.