Galatians 4:6 states, “And because you are sons, God has sent the Spirit of his Son into our hearts, crying, ‘Abba! Father!’”
The verb krazō (“to cry out”) is used 55 times in 54 verses in the NT, often in a literal fashion. For example, Peter cried out for Christ to save him from sinking in the water (Matt 14:30), people cried out when possessed by demons (Mark 5:5; 9:26), and people cried out for the death of Christ (Mark 15:13). Having some literal examples in hand, we feel this verb’s intensity when it is used of the Spirit’s action inside of someone’s heart. The Spirit cries out, “Abba! Father!” What is going on here? Are we supposed to somehow literally hear these words within our hearts to know that we are the sons of God?
What Paul is describing is a figurative crying out—it is not something audible. And yet, it takes place, and we know God is our Father because of this cry.
If God has truly saved us, and if we are truly His children, the Spirit does a work within us whereby we are confirmed that we are God’s children. This work is described here as the Spirit’s crying out within us that God is our Abba and Father. Abba was a term not usually used of God and stressed the intimate relationship of child and father. Father emphasizes this relationship as well. While the Spirit may not audibly make this cry within us, we know when this cry takes place because we ourselves are the ones saying that God is our Father, all at the Spirit’s prompting. In a parallel passage, Paul says that “you have received the Spirit of adoption as sons, by whom we cry, ‘Abba! Father!’” (Rom 8:15 ESV). This process is described in this way: “The Spirit himself bears witness with our spirit that we are children of God” (Rom 8:16 ESV).
Comparing Scripture to Scripture with these two passages, we could say that one of the ways whereby we know that we are the children of God is this—God has placed the Spirit in us to bear witness to our spirits in such a way that moves us to cry with certainty to God that He is our Father.
To put it more simply, Galatians 4:6 provides us with part of the answer to the question, How can you know you are a Christian? Well, to be a Christian is to be a son of God, and we know we are the sons of God when the Spirit who lives within us prompts us to call upon God as our Father.