His subject is apostasy. They considered the Gospel, and even made a profession of faith, then turned from the truth of Christ's effectual remission of sin. It's a particular kind of sin. It isn't moral failure but doctrinal corruption. He called it "willful" or voluntary, thoughtful and purposeful. They heard, but as those in chapter 5, they remained "dull of hearing" and "unskillful in the word of righteousness".