Over there in the hot sands of the peninsula, that means being a follower of Jesus. Hence, if he denies it, he denies being a follower of Jesus, meaning he denies that Jesus is who He said He was.
This isn't difficult.
Tell you what. Come Judgment Day, why don't you approach God's throne with this kind of logic and see how far you get with it?
Oh, good. We've got God's deputy here to straighten this whole thing out. Glad that somebody around here speaks for God.
So he is guilty of the sin of lying. But if in his HEART he is a follower, he is still saved.
Let's look at the counter position: what if a Christian gang of thugs were asking atheists, at the point of a gun, if they were Christians. If the atheist lies and says yes, is that atheist saved? I think not.