Jerome, Commentary on Galatians, [J.P. Migne, Editor]. Patrologiae Tomus XXVI. Paris: D’Ambroise, Pres La Barriere D’Enfer, ou Petit-Montrouge, 1845. pp. 307-438.
Galatians 4:6 speaks of God sending the Spirit of His Son to make us call out to the Father. Jerome notes that Paul speaks in Romans 8:14 of the Spirit of the Son, in 1 Cor. 6:19 or fhe Holy Spirit, and elsewhere of the Spirit of the Father (Jerome, Galatians, 373). The apostle is decidedly Trinitarian. Jerome further refers to Psalm 51, observing that the Lord is called on to create a new heart and spirit in David. God’s will is to make his people call out to him.
It is particularly important to Jerome, as to Paul, that Jesus has provided adoption for his people, which is the only thing that makes us able to call out to Him. Without the adoption, we are not the sons of God (Jerome, Galatians, 374).
Because of the adoption in Christ, in verse seven we learn that we are not slaves but sons, therefore heirs of Christ. Jerome notes that this means we are heirs of both the Father and the Son. We are then placed on par with God the Son, Jesus (Jerome, Galatians, 374). This is the great hope and confidence of the Christian.