But Jesus wasn't just "a nice guy"
He is God, who came to earth in human form to save humanity from the punishment that we deserve.
While he was in human form, for 33 or so years, he did do many miracles and preached the gospel, but His real work was dying on the cross and descending into hell to pay the penalty for our sins. He defeated Satan's accusations of us.
There are many religions who acknowledge Jesus as a "nice guy" but Christianity acknowledges Him as The Promised Messiah.