Christianity is about a personal relationship with God made possible by the act of 'Jesus, who was both man and God'.
Christians experience (spiritually) the person of Jesus. Jesus is very real to those who know Him. Once having experienced Jesus, it is impossible to deny him.
Just as it is impossible to deny any experience.
Those who do find it possible to deny thier own experience of Jesus, are known as backsliders and therefore condem Him to be crucified once more.