Because of the wealth of historical evidence on this subject.
If Jesus wasn't nailed to a stick and resurrection, then Christianity has no rational basis.
Fortunately, there's a lot of early contemporary Christian and non-Christian sources that say he was crucified. And I believe there's a good case that even without the Christian sources to say he was resurrected (if you believe in the resurrection you're a Christian by definition).
The non-christian evidence says that 2000 years ago a man called Jesus lived and was crucified and a few days later his tomb was found empty. First generation Christians started claiming that he had been resurrected. They believed this enough to die for it. Remember these were people who were in a position to know 100% whether or not Jesus rose from the dead, they wouldn't have died if they knew it wasn't true. So, you have to concede that at the very least early Christians believed this to be true.
On top of the non-Christian evidence, there's also the 4 gospels (or biographies of Jesus' life) found in the bible. These can be dated very early to the events in question, and many historians have claimed that even with conservative estimates of the dating it would be unprecedented for legend to spring up in the short time between the events and when they were written. There were other Christian writings at the time that have not been added to the New Testament, for example the Gospel of the Hebrews says similar things
A book I found helpful is called "The Case for Christ", it was written by a former Athiest laywer who became a Christian due to the evidence. It recounts his investigation. It's good because it's at a level everyone can read and understand, though they're higher level stuff if you want to read it.
I've also posted a youtube link about the extra biblical evidence there is.
If you want to investigate the historical evidence, I daresa