Albert Barnes on Galatians 3:26

For ye are all the children of God, etc. All who bear the Christian name--the converts from among the Jews and Gentiles alike. Jn 1:12. The idea here is, that they are no longer under tutors and governors; they are no longer subject to the direction and will of the paedagogus; they are arrived at age, and are admitted to the privileges of sons. Gal 4:1. The language here is derived from the fact, that until the son arrived at age, he was in many respects not different from a servant. He was under laws and restraints, and subject to the will of another. When of age, he entered on the privileges of heirship, and was free to act for himself. Thus, under the law, men were under restraints, and subject to heavy exactions. Under the gospel, they are free, and admitted to the privileges of the sons of God.

(a) "children of God" Jn 1:12, 1Jn 3:1,2