Mat 26:34 Verily I say unto you, That this night, before the cock crows, you shall deny me thrice.

Christ does not see doubt and even rejection as a "sin," but simply as a predictable fact of our human nature. In the previous verse he said that both he and his followers must fall, as predicted, but that then they would rise.

For Christ, progress is not a straight line, but a cycle that we go through. We come to believe, but that belief creates doubt and that doubt itself allows us to renew our faith. No matter how much you or I believe at this moment, Christ is predicting that we will eventually forget that belief or worse, come to deny it. However, if we recognize what we are doing, the denial in itself can cause us to come to our senses.

"Deny" is from aparneomai (aparneomai), which means "to deny utterly," "to refuse," "to reject," and "to deny."