I know if you die defending the faith you are considered a martyr, but if someone says they will kill you if you don't renounce God, and you renounce just to save yourself, is that a sin? Especially if it's only words.
Realize what you are asking. You are asking if it is a sin to deny God. Of course, it is! This is what sin is: denying who HE is and preferring our own agenda to His.
In Colorado a few years ago a killer was loose in a high school and asked a young girl if she was a Christian. She knew he was deranged and could kill her. But she would not deny you allegiance to Christ and said yes. He immediately shot and killed her. SHE is now a martyr in heaven.
If we are not willing to risk our hides for Him, can we call ourselves His followers? If not; then we need to spend more time before the tabernacle, getting to know Him.
Fr. Vincent Serpa, O.P.