A few thoughts about this verse, not to take issue with anyone, but to say I think there’s more theology in the Bible than we sometimes think.
I agree with most Christians that the fully developed doctrine of the Trinity needed to be defined by the church because the Bible is not written in the form of sophisticated and exact theological definition.
But I don’t agree that the verse above is particularly ambiguous or that it can be interpreted reasonably or correctly with non-Trinitarian assumptions.
For starters, the verse is part of the epistle to the Galatians and that letter exists in the larger context of a classically Jewish, Biblical and emerging Christian context.
The key concepts in the verse are part of the larger Biblical conversation. If that conversation is referenced, there is no reasonable option for interpreting the verse in any way except in a way that leads to the Trinity.
When Paul writes the Galatians, the term “God” is specific to the God of the Bible revealed by Jesus. This includes the identification of Jesus with God himself, which is not a matter of interpreting texts, but a foundational fact of Jesus as understood by early Christianity. (See the work of Larry Hutardo on early Christian devotion to Jesus.) There is no “developing tradition” of Jesus’ relationship to God. There is theological clarification of an existing understanding that God had come in to Israel and the world in Jesus. The result is devotion to Jesus as God.
Galatians also exists in the Biblical understanding that the Spirit of God is God himself. There is no impersonal spirit that would be used in the sense of God pouring “the Spirit of His Son” into the hearts of believers. The Spirit of God is identified so closely with God that it would considerably odd to hear someone make the claim that “the Spirit of His Son” would be an impersonal substance or a spirit separate from God Himself.
Does this imply a fully Trinitarian view of the Spirit? It’s not the language of the creeds, but it is the language of scripture, and that language is unambiguous in this context.
Likewise, the concept of Son in early Christianity, particularly in Galatians, was clearly more than the concept of Messiah. It may not have taken on all the language of later sophisticated Trinitarianism, but the unique relation to God and action on behalf of God is central.
Notice Galatians 2:20:
I have been crucified with Christ. It is no longer I who live, but Christ who lives in me. And the life I now live in the flesh I live by faith in the Son of God, who loved me and gave himself for me.
The indwelling Christ/Messiah is the Son of God, but is the one who loved and gave. The language of loving and giving is the language of John, which that Gospel traces to Jesus himself.
15 â€œIf you love me, you will keep my commandments. 16 And I will ask the Father, and he will give you another Helper, to be with you forever, 17 even the Spirit of truth, whom the world cannot receive, because it neither sees him nor knows him. You know him, for he dwells with you and will be in you. 18 â€œI will not leave you as orphans; I will come to you. 19 Yet a little while and the world will see me no more, but you will see me. Because I live, you also will live. 20 In that day you will know that I am in my Father, and you in me, and I in you.
Unless we want to participate in some historical/theological gymnastics, we must say that some form of the language of Jesus recorded in John- language of the Son who is uniquely divine and the Spirit of that Son being given to disciples- is at least somewhat familiar (in some form) to Paul when he writes Galatians 4:6.
I would further argue that the action here is not only the God of the Bible manifested as Spirit and Son, but I believe the natural understanding of the passage is…
1. That this God is a unity, acting in one purpose.
2. That polytheism is not an option.
3. That Arianism cannot fairly resolve this statement without doing damage to the concept of the Spirit as uniquely coming from God and without reinterpreting both the witness of the Gospel of John and early Christian devotion along the lines of its own presuppositions.
4. That no other interpretation of Galatians 4:6 is consistent with all the context available except one God acting as three persons. While the full nature of this relationship is clarified by the full doctrine of the Trinity, I do not believe the passage, in its proper context, in any way leads the reader away from the doctrine of the Trinity.