6. because ye are sons--The gift of the Spirit of prayer is the consequence of our adoption. The Gentile Galatians might think, as the Jews were under the law before their adoption, that so they, too, must first be under the law. Paul, by anticipation, meets this objection by saying, YE ARE sons, therefore ye need not be as children (Ga 4:1) under the tutorship of the law, as being already in the free state of "sons" of God by faith in Christ (Ga 3:26), no longer in your nonage (as "children," Ga 4:1). The Spirit of God's only Begotten Son in your hearts, sent from, and leading you to cry to, the Father, attests your sonship by adoption: for the Spirit is the "earnest of your inheritance" (Ro 8:15, 16; Eph 1:13). "It is because ye are sons that God sent forth" (the Greek requires this translation, not "hath sent forth") into OUR (so the oldest manuscripts read for "your," in English Version) hearts the Spirit of His son, crying, "Abba, Father" (Joh 1:12). As in Ga 4:5 he changed from "them," the third person, to "we," the first person, so here he changes from "ye," the second person, to "our," the first person: this he does to identify their case as Gentiles, with his own and that of his believing fellow countrymen, as Jews. In another point of view, though not the immediate one intended by the context, this verse expresses, "Because ye are sons (already in God's electing purpose of love), God sent forth the Spirit of His Son into your hearts," &c.: God thus, by sending His Spirit in due time, actually conferring that sonship which He already regarded as a present reality ("are") because of His purpose, even before it was actually fulfilled. So Heb 2:13, where "the children" are spoken of as existing in His purpose, before their actual existence.
      the Spirit of his Son--By faith ye are one with the Son, so that what is His is yours; His Sonship ensures your sonship; His Spirit ensures for you a share in the same. "If any man have not the Spirit of Christ, he is none of His" (Ro 8:9). Moreover, as the Spirit of God proceeds from God the Father, so the Spirit of the Son proceeds from the Son: so that the Holy Ghost, as the Creed says, "proceedeth from the Father and the Son." The Father was not begotten: the Son is begotten of the Father; the Holy Ghost proceeding from the Father and the Son.
      crying--Here the SPIRIT is regarded as the agent in praying, and the believer as His organ. In Ro 8:15, "The Spirit of adoption" is said to be that whereby WE cry, "Abba, Father"; but in Ro 8:26, "The SPIRIT ITSELF maketh intercession for us with groanings which cannot be uttered." The believers' prayer is His prayer: hence arises its acceptability with God.
      Abba, Father--The Hebrew says, "Abba" (a Hebrew term), the Greek, "Father" ("Pater," a Greek term in the original), both united together in one Sonship and one cry of faith, "Abba, Father." So "Even so ('Nai,' Greek) Amen (Hebrew)," both meaning the same (Re 1:7). Christ's own former cry is the believers' cry, "Abba, Father" (Mr 14:36).

JFB.