What are all the things that Christians have to buy into BEFORE trusting anything about any god?

Not nearly as much as one would need to rewrite or ignore to deny God. If you deny that Christ existed you need to explain how the Christian church arose at the time of his crucifixion and resurrection. Those who claim Christianity and the New Testament were the creation of Constantine, have a lot of explaining to do about the fact that the majority of the NT was written by the end of the first century AD and the writings and doctrines of the first century church fathers was remarkably consistent with current doctrine and beliefs. Sure there are certain practices and dogma that have crept in over the centuries. The important thing is that the foundational elements of God's love and grace, Christ's position as God incarnate and sacrifice for our sins and path for our salvation, and the call for us to live Godly lives and love others as ourselves is so well established historically and Biblically, that you would have to be blind not to see it. I find people who try to deny things like the Exodus so disturbing. How then does one explain the birth of the Hebrew nation with all of their customs, festivals, artifacts, and writings that attest to this event? Something happened then, and something happened in the early first century to spawn Christian beliefs and practices. What was it if not what was written by those closest to the event?