All of God's previous covenants were never fulfilled — the angels fell, Adam sinned and the Israelites couldn't follow the Law — until Christ came and completely fulfilled the Old Covenant. Jesus established the New Covenant — one where God gives us the grace to follow His commands. When we "drift away," as the writer of Hebrews says, it is not toward immorality, but back toward an old covenant works-based mentality. Our good works cannot add anything to Christ's finished work on the Cross. Works are a result of God's grace. They do not make us more holy or more acceptable to God.