It boggles my mind that people have trouble understanding what Christianity is. It’s the Gospel. Everything else- the history, philosophy, art, dogmas, heresies- is all fluff and ultimately irrelevant. If it includes the Gospel, it’s Christianity, and if it doesn’t it’s not. The name is not accidental.
tl;dr- All people sin, sinners go to Hell, believe in Jesus and be saved.
Why believe? Coming back to life is an unusual talent for a mere historical figurehead. We know he was resurrected because the history books say so, plus the authorities of the time were thoughtful enough to torture the eyewitnesses to death and observe they still wouldn’t recant. That’s as certain as you can be about anything.
The rest, as I mentioned, is fluff. (Copied from elsewhere because apparently this does not go without saying.)