The lines of descent from Sarah and Hagar are fluid. In other words, physical sons of Sarah can become Ishmaelites through unbelief. Physical sons of Hagar can come in faith to Christ and be adopted into Israel. That which God uses to determine which way it goes is faith which He imparts as a sovereign gift—faith now, faith alone, and faith forever. For this orthodox emphasis, some of us who hold firmly to this truth have been accused by some theologians—whose subtlety is wondrous to behold—of denying sola fide. How much there is to this foolishness we will see soon enough.
Stand fast therefore in the liberty wherewith Christ hath made us free, and be not entangled again with the yoke of bondage. Behold, I Paul say unto you, that if ye be circumcised, Christ shall profit you nothing. For I testify again to every man that is circumcised, that he is a debtor to do the whole law. Christ is become of no effect unto you, whosoever of you are justified by the law; ye are fallen from grace. For we through the Spirit wait for the hope of righteousness by faith. For in Jesus Christ neither circumcision availeth any thing, nor uncircumcision; but faith which worketh by love. Ye did run well; who did hinder you that ye should not obey the truth? This persuasion cometh not of him that calleth you. A little leaven leaveneth the whole lump (Gal. 5: 1-9)
We have just finished learning that sons of unbelief are sons of Hagar. Sons of faith are sons of Sarah—they are children of the promise. But to start with promise and try to finish with human effort (Gal. 3:1) is to fall into apostasy. This is how sons of Sarah over time became sons of Hagar. Now, Paul says, you Galatians must stand fast in your liberty (v. 1). Paul has just finished saying that the Galatians are children of the free woman (4:31). But now, in the next breath, he says that if they accept circumcision, Christ will profit them nothing (v. 2). Back to Hagar. Why is this? If a man accepts circumcision (in this context), he is obligated to keep the entire law on his own steam (v. 3). Christ is ineffectual for him. Those who seek justification through the law have fallen from grace (v. 4). But we (through the Spirit) wait for the hope of justification by faith (v. 5). In Jesus, it is neither circumcision or uncircumcision, but rather faith working in love (v. 6). The Galatians had started their race well—who was it that got in their way so that they did not obey the truth (v. 7)? This opinion they were entertaining was not from the Holy Spirit (v. 8). A little works leavens a big lump of grace (v. 9).
What is the "lens of the covenant?" Paul is confronted with a pastoral problem in the churches of Galatia. He is not confronted with a logical problem in systematics class in a seminary. People who fall away from the church are falling away from grace. They are falling away from Christ. We do not know if they have fallen away from the eternal secret decree of election. Of course not. If we ask if someone can fall away from the secret decree of election, the answer is obviously no. But that is the logic problem. If all dogs have four legs, and this is a dog, then this has four legs. If all the secret elect are predestined to glory, and this person is among the secret elect, then this person is predestined to glory. Great, and amen. But we do not have possession of the contents of the secret decree (Dt. 29:29). We are to walk by faith and not by sight. We do see the covenant, and the presence of Christ in it. That is, we see Christ in the covenant with the eyes of faith. But if we start adding other ways to justify ourselves before God, then we have fallen from grace because of our manifest unbelief.
The issue is not this particular passage or that one. The issue is not whether our handling of a passage could be tighter or not. The issue is how we think, how we respond. Do we let Scripture define everything in our lives, or do we try to have our lives (and our understanding) shape the Scriptures?
There are only two means of justification, and one of them doesn’t work. One is self-justification, and the other is Christ-justification. If anyone is circumcised (with this idol of self-justification in his heart), then is bound over to self-perfection, which is impossible. But the problem is the idol, not the circumcision. The same thing goes for baptism, lack of baptism, Westminster-confessing, lack of it, memorizing the catechism, hating catechisms, and so on. This is not complicated. At the end of all the rationalizations, it is this question: self? or Christ?
Those who are true sons of Sarah are those who through the Spirit wait for a future hope—a future justification, a future righteousness (v. 5). How do we wait for this? We wait for it by faith, in faith, through faith. There is a sense in which the elect are already justified at their conversion, but there is this sense here also. We look forward to it, and we do so by faith. But note that here Paul is speaking of justification as some sort of future event. The word has more than one scriptural meaning.
It is hard to run, and easy to trip someone who is running. It is easy to persuade contrary to the teaching of the Spirit. Leaven works through the whole load silently, easily, and readily. It does not need to be coaxed. And the leaven of works does corrupt grace. The human heart takes naturally to self-flattery, and loves any religion that will help this process of flattery along. The problem with the critics of the "objective covenant" is therefore not that they are too critical and cautious. The problem is that they are not nearly cautious enough. We are justified by faith, through faith in Jesus. We are not justified through faith in justification by faith.