Albert Barnes Commentary Galatians 4:6

"And because ye are sons, God sent forth the Spirit of his Son into our hearts, crying, Abba, Father." — Galatians 4:6 (ASV)

And because you are sons. As a consequence of your being adopted into the family of God and being regarded as His sons, it follows as a part of His purpose of adoption that His children will have the spirit of the Lord Jesus.

The Spirit of his Son. This refers to the spirit of the Lord Jesus—the spirit that animated Him or that He demonstrated. The idea is that just as the Lord Jesus was enabled to approach God with the language of endearment and love, so they also would be. He, being the true and exalted Son of God, had the spirit appropriate to such a relationship; they, being adopted and made like Him, have the same spirit.

The "spirit" referred to here does not mean, I believe, the Holy Spirit as such, nor the miraculous endowments of the Holy Spirit. Rather, it is the spirit that made them like the Lord Jesus—the spirit by which they were enabled to approach God as His children and use the reverent, tender, and affectionate language of a child addressing a father.

This is the language Christians use when they have evidence of adoption: the expression of the warm, elevated, and glowing emotions they experience when they can approach God as their God and address Him as their Father.

Crying. That is, the spirit thus cries, pueuma krazon. Compare to Romans 8:26 and Romans 8:27.

In Romans 8:15, it is, wherewith we cry.

Abba, Father. (See the notes on Romans 8:15).

It is said in the Babylonian Gemara, a Jewish work, that slaves were not permitted to use the title "Abba" when addressing the master of the family to which they belonged. If so, then the language that Christians are represented as using here is the language of freemen, and it denotes that they are not under the servitude of sin.