What events recorded in Scripture must be historical for you to affirm the truthfulness of Christianity?
For myself the answer is short and simple: the resurrection of Christ. I think all of Christianity stands or falls on whether God really raised Jesus from the dead.
To further the conversation, here are a few events of the Bible that I don’t think are historical: Genesis, Exodus, etc. A more important event that I think is probably not historical is the virginal conception of Jesus.