What does the Bible say about the sin of turning away from the gospel?

In Galatians 4:17-20, the Apostle Paul expresses his bewilderment at the quick departure of the Galatians from the gospel of free grace. This departure is not merely a doctrinal shift but a turning away from Christ Himself. Paul refers to this as the perplexing sin because it is startling for those who have been called by God's grace to abandon the very cornerstone of their faith. This reflects a deeper issue of not merely sinning against God's law but rejecting the gospel that offers salvation through Christ alone.