Most people don't realize this but Paul is the earliest known Christian writer and the earliest surviving source for the Gospel. His letters also independently corroborate not only Jesus' existence as a real person (in addition to secular sources), but also that Jesus' close followers genuinely believed they met Jesus' resurrected form. Since Paul's witness is dated to within 3-5 years of Jesus' death, this also shows that the Gospel didn't develop as a myth/legend, but as something people genuinely believed who had personally met Jesus. It's a fascinating story of a Jewish religious scholar who hallucinates on the road to Damascus, has a sudden complete change of heart, and ends up transforming Christianity from a local Jewish cult into a worldwide religion.
Another fascinating topic in biblical study is the criterion of embarrassment, where the early Christian writings left in bizarre and unflattering events that members of a cult would generally leave out. The most obvious example is the crucifixion itself (considered by Jews to be extremely shameful and cursed), the repeated unflattering presentation of the disciples (portrayed as regularly confused, lacking in faith, petty about status, falling asleep at critical moments, even rejecting Jesus at the end), even Jesus' own despair when he was publicly humiliated and executed, crying out asking God why he was forsaken. This is in contrast to Islam, which has Jesus rescued and replaced at the moment of execution.