Paul himself said so, in 1 Corinthians 15.
4He was buried, and he was raised from the dead on the third day, just as the Scriptures said. 5He was seen by Peter and then by the Twelve. 6After that, he was seen by more than five hundred of his followers at one time, most of whom are still alive, though some have died. 7Then he was seen by James and later by all the apostles. 8Last of all, as though I had been born at the wrong time, I also saw him.
“Something happened on Easter Day which made Christ more alive on the streets of Jerusalem forty days after his crucifixion than on the day of His Triumphant Entry. A false report might last forty days but the church which was founded on a Risen Christ has lasted for nineteen centuries, producing generations of the race’s finest characters.” Ralph W. Sockman
"Twenty centuries now. And stronger than ever."