"You have been cut off from Christ, you who would be justified by the law, you have fallen from grace." Galatians 5:4
Some of Paul's most severe language is found in his letter to the Galatians. This letter is also the place where some of his clearest language on perseverance and falling away can be found. This verse from chapter 5 is one of those places. To say that one has been cut off from Christ implies that one is joined to Christ in the first place. The Galatian Christians, having been justified by grace, joined to Christ, and received the Spirit, are now trying to maintain their standing with God on the basis of Torah observance. Instead of moving forward in salvation history as God has designed, they are seeking to move back to a previous covenantal administration which is a denial of the gospel of Christ. The point is that Paul is dealing with those who have the Spirit (Gal 3:3) and have been joined to Christ, and yet he sees it as a real possibility that they could be cut off from Christ and lose their justification. Paul does not see them as being without hope; that's why he wrote the letter. He is quite clear, though, that despite being joined to Christ, their final perseverance is in jeopardy.