Because he was reported to have appeared to witnesses and spoke with them days after the crucifixion.
Jesus was crucified in public. So either he actually wasn't crucified, or everyone couldn't tell that the wrong guy was on the cross due to a clever bait-and-switch. Maybe possible, especially since Jesus was beaten bloody prior to crucifixion.
Well, there's already been a lot of discussion about the crucifixion of Christ here. https://en.wikipedia.org/wiki/Crucifixion_of_Jesus
Some may laugh at the crucifixion of Christ and claim that it's an easy passport to heaven. Sin at will, still go to heaven. Ultimately, a person saved by grace doesn't sin freely even though being human, they still do sin.
If you want to sin freely and still go to heaven, be a moslem. When moslems cheat, kill and have sex with kids, people know that they did so following their prophet's past behavior. When Christians sin at will, we know that they are not following Christ because Christ never once in his life sinned.