Jan Lambrecht, «Abraham and His Offspring. A Comparison of Galatians 5,1 with 3,13», Vol. 80 (1999) 525-536
Just as after the Abraham passage of 3,6-12 Christ is mentioned in 3,13 quite unexpectedly, so also after 4,21-31, Pauls so-called allegory which deals with the wives and sons of Abraham, the sudden statement about Christ in 5,1 cannot but surprise the reader. Although the word order differs, both vocabulary and content of parts of 3,13a and 5,1a are identical or at least similar. Abrahams faith was already, by way of anticipation, Christian faith. Moreover, "those of faith" in 3,7 and 9 implicitly are believers in Christ. This also applies to 4,26. The children of "the Jerusalem above" are free because they belong to Christ, even if in v. 26 this is not (yet) explicitly stated. Therefore, a seemingly brusque transition from the Abraham text or the allegory to Christ should not disturb the reader too much.
his family. That Genesis material provides him not only with an illustration. A promise was made to Abraham and he believed God; this faith was reckoned to him as righteousness. According to Paul, Abrahams faith was already, by way of anticipation, Christian faith. Moreover, for Paul oi( e)k pi/stewj in 3,7 and 9 implicitly are believers in Christ. This also applies to 4,26. The children of h( a!nw )Ierousalh/m are free because they belong to Christ, even if in v. 26 this is not (yet) explicitly stated. God promised an inheritance to Abraham. Through their belonging to Christ Christians have become, not by law but according to that promise, the heirs (cf. 3,29)22.
Abrahams offspring is Christ; that offspring at the same time consists of all those who have faith in Christ23. Therefore, a seemingly brusque but easy transition from Abraham to Christ should not disturb the reader too much.