Gal.4:6 – Because you are sons, God has sent the Spirit of His Son into our hearts, crying out, “Abba! Father!”

In this passage in Galatians Paul is talking about growing up in Christ. He says that an indication that we are still children is that we are still encumbered and even fixated on the law. Of course Paul was talking about the law of Moses but even us gentiles have a battle in this area. All humans have concocted some version of the law in order to direct their lives. The problem is that we always fall short and never attain to the standards of whatever law we enact. Paul tells us the solution here, the Lord sends the Spirit of His Son into our hearts so that we become sons of God. The indication of this is that the Spirit in our heart does truly cry out Abba Father. Here is how Barclay describes this.

“Paul says that when the Galatians – and indeed all men and women – were mere children, they were under the tyranny of the law; then, when everything was ready, Christ came and released them from that tyranny. So, now they are no longer slaves of the law; they have become heirs and have entered into their inheritance. The childhood which belonged to the law should be past; the freedom of adulthood has come.
The proof that we are God’s children comes from the instinctive cry of the heart. In our deepest need, we cry: ‘Father!’ to God. Paul uses the double phrase, ‘Abba! Father!’ Abba is the Aramaic word for father. It must have often been on Jesus’ lips, and its sound was so sacred that the original language was retained. This instinctive cry of the heart Paul believes to be the work of the Holy Spirit. If our hearts cry out in this way, we know that we are God’s children, and all the inheritance of grace is ours.
For Paul, those who governed their lives by slavery to the law were still children; those who had learned the way of grace had become mature in the Christian faith.”

This passage is describing the amazing freedom that sons and daughters of God now experience. Before Christ came we were all laboring trying to live the standard of the law in our lives, now the Spirit is inside of us causing us to live a totally different kind of life. The Spirit is in us, and God is our Father. The cry of Abba, the love language of the Spirit, is now inside of us. We are free from the bondage of rules but walk in purity and holiness by the Father’s love.