Abraham has only one seed, Christ (v. 16). Hence, to be Abraham's seed we must be of Christ, be a part of Christ. Because we are one with Christ, we too are Abraham's seed, heirs according to promise, inheriting God's promised blessing, which is the all-inclusive Spirit as the ultimate consummation of the processed God, who is our portion. Under the new testament, the believers as God's chosen people, being sons of full age, are such heirs, not under law but in Christ. Like Ishmael (Gal. 4:23), the Judaizers, who remained under law and kept themselves apart from Christ, were Abraham's descendants according to the flesh; they were not like Isaac (Gal. 4:28), who was Abraham's heir according to promise. But the believers in Christ are such heirs, inheriting the promised blessing. Hence, we should remain in Christ and not turn to the law.
Since the law is unable to give us life (v. 21), it cannot produce the sons of God; but the Spirit, who is received out of faith (v. 2) and who gives us life (2 Cor. 3:6), can. The law kept God's chosen people under its custody until faith came (v. 23). Faith in Christ, who is the all-inclusive life-giving Spirit, makes God's chosen people Abraham's seed as "the stars of the heaven" (Gen. 22:17) according to God's promise.