Commentaries:
Robertson's Word Pictures (NT)
Because ye are sons (oti este uioi). This is the reason for sending forth the Son (Galatians 4:4 and here). We were "sons" in God's elective purpose and love. oti is causal (I Corinthians 12:15; Romans 9:7).
The Spirit of his Son (to pneuma tou uioi autou). The Holy Spirit, called the Spirit of Christ (Romans 8:9 f.), the Spirit of Jesus Christ (Philippians 1:19). The Holy Spirit proceeds from the Father and from the Son (John 15:26).
Crying, Abba, Father (krazon Abba o pathr). The participle agrees with pneuma neuter (grammatical gender), not neuter in fact. An old, though rare in present as here, onomatopoetic word to croak as a raven (Theophrastus, like Poe's The Raven), any inarticulate cry like "the unuttered groanings" of Romans 8:26 which God understands. This cry comes from the Spirit of Christ in our hearts. Abba is the Aramaic word for father with the article and o pathr translates it. The articular form occurs in the vocative as in John 20:28. It is possible that the repetition here and in Romans 8:15 may be "a sort of affectionate fondness for the very term that Jesus himself used" (Burton) in the Garden of Gethsemane (Mark 14:36). The rabbis preserve similar parallels. Most of the Jews knew both Greek and Aramaic. But there remains the question why Jesus used both in his prayer. Was it not natural for both words to come to him in his hour of agony as in his childhood? The same thing may be true here in Paul's case.
Other Robertson's Word Pictures (NT) entries containing Galatians 4:6:
Matthew 1:1
Mark 14:36
Romans 8:15
Galatians 4:9
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