Ambrosiaster explains this keenly, raising his thought to the promise to Abraham: “It was fitting for the Savior to be under the law as Abraham’s son according to the flesh.
The One Who, according to the promise to Abraham, was to come for the justification of the nations by faith had to appear circumcised. He bore upon Himself the sign of the one to whom He had been promised.
After Him no one should be circumcised. For that sign passed from generation to generation only until the promised Restorer of faith came. By that faith Abraham was justified in uncircumcision, receiving circumcision only as a sign of faith.
This faith justified him and was to justify all, so that circumcision would no longer be needed by believers. Abraham was circumcised because he believed that he would have a Son in Whom all nations would be blessed.
Therefore this sign had meaning only until the Promised One came.”
Let us add that circumcision is the root of the law. Where circumcision is, there is the whole law.
“That we might receive adoption.” Here the Apostle discusses the blessings of the Incarnation of the Son of God only in relation to the comparison he has set forth. His thought is to impress upon them that the law has already ceased and has no obligatory force for believers, according to the very appointment of God the Father.
We are sons by God’s election. Yet until the term appointed by our Father, God, we were placed under the law and lived beneath its yoke as though slaves. The term ended. The Father Who placed us under this tutelage sent the Son to dismiss the tutor and bring us into the rights and freedom of sons.
“By saying, ‘that we might receive adoption,’ the Apostle showed that it is the portion of inheritance assigned to us from of old and due to us. We had not received it before because of the infancy of our mind” (Oecumenius; the same is found in Saint Chrysostom and Saint John of Damascus).
The Apostle seems to set forth only the external course of events in the divine economy of our salvation, feeding with milk and not with solid food. Yet the following words (Gal 4:6-7) show that he himself looked more deeply here, as everywhere. He also disposed his readers to enter the same depth after him.
The Son of God came and made us sons. He did not merely declare us sons, but made us so. He gave us authority to become children of God, because in Him we are born of God.
Here sonship is not nominal, but substantial. If we place this thought in relation to the law, we get this course of thought: the law, as a guardian, disciplined outwardly by restraining impulses toward sin.
But when the Son of God came, He bestowed the Holy Spirit. By regenerating believers into new life in Christ Jesus, the Spirit cut off the very impulses and destroyed sin within by grace.
After this, the external guardianship of the law falls away by itself. The law ceases, as the chrysalis breaks when the butterfly has formed and come alive within. The Apostle’s following words about the Holy Spirit show that, in the essence of the matter, this is his main thought, hidden beneath the external history of the event.