If he proclaims to be a Christian, as he says he does, anything other than faith in Christ should not have crossed his mind nor slipped past his tongue.
IMHO, that "verbal slip" was confirmation of his true heart.
Furthermore, if he can be rattled that easily into "misspeaking" the polar opposite of what he claims to believe, then what other "revelations" might his words uncover?
I have never inadvertently referred to my Hindu faith, my Buddhist faith, or my Muslim faith. I am a Christian, and I have never thought those phrases before, let alone said them.
The man was raised to be a Muslim. He says he no longer is. I don’t believe him.
From your lips to Gods ears,