Atheists why were they fearless for God?

If Jesus really wasn’t the Son of God, and he didn’t really rise from the dead 3 days after his crucifixion, why did a group of men and women that had been hiding in an attic for those 3 days suddenly come out and start preaching about this man in a way that they were willing to suffer gruesome deaths for?(And don’t give me anything about mankind needing an explanation in their minds, any simple “explanation” would be quickly dropped when a man is about to be sawed in two, stoned, crucified, burnt alive, or beheaded