The New Testament writers universally claim that Jesus rose from the dead, and they report that he was seen by numerous eyewitnesses. Not only so, but history tells us that some of these same eyewitnesses subsequently died for their faith.
They were that sure that Jesus rose from the dead.
Were they suffering from cognitive dissonance? Did they deceive themselves into imagining this? Was it likely they made the story up?
Not if they were willing to die for it. As the saying goes, you don’t die for a lie....