Note 2 at Gal. 4:9: Paul is writing this letter to turn the Galatian Christians away from trusting in the Old Testament law for justification (see INTRODUCTION TO THE EPISTLE OF PAUL TO THE GALATIANS, p. 1050). So, in this verse, when he refers to turning, "to the weak and beggarly elements, whereunto ye desire again to be in bondage," he is referring to turning to the law. In doing so, Paul is likening the Galatians' departure from the gospel, to them turning back to the idolatry they practiced before their salvation (v. 8). Indeed, legalism is very similar to idolatry.

Just as with the worship of idols, legalism assumes God is a wrathful God who has to be appeased by our efforts. In contrast, Christianity presents a merciful God who has personally paid for our sins and offers us salvation as a gift. Idolatry has countless rituals and so does a legalistic interpretation of the Old Testament law. Legalism is bondage just as all false religions are. The law is "weak," i.e. powerless to save, just as all idolatry is powerless. The same Greek word that was translated "weak" here was also translated "feeble, impotent, sick, and without strength." The law is "beggarly" in the sense that it is totally inadequate. This same Greek word was translated "poor" 30 times in the New Testament.

So the law in its proper place (see note 3 at Rom. 3:19, p. 757) and used for its proper purpose (see note 4 at Rom. 3:19, p. 757) is good (see note 14 at Rom. 3:31, p. 760). But any attempt to mix the gospel (see note 5 at Acts 20:24, p. 698) and law together is bondage (see note 6 at Gal. 1:6, p. 1053).