Gordon Fee on the Spirit Crying "ABBA!"

"Here is the ultimate evidence that we are God's own children; we address God with the same term of intimate relationship that Jesus Himself used. We are not slaves but children. The Spirit has taken us far beyond mere conformity to religious obligation. God Himself ... has come to indwell His people and He has sealed that relationship by given to them the language of His Son, the language of personal relationship. For Paul and us this is the ultimate expression of grace".

Gordon D Fee - "Pauline Christology - An Exegetical/Theological Study" - Hendrikson (2007) - (p219).