you received the Spirit of adoption to sonship: According to Greco-Roman customs of adoption, a man could adopt a son and give that child the same legal rights as a natural son. This practice included the highest levels of society, even the imperial family. For example, the Roman ruler Julius Caesar adopted Octavian as his heir. Octavian later ruled the Roman Empire under the name Augustus.
Paul’s understanding of adoption is also rooted in the Old Testament and Jewish tradition. In the Scriptures, God describes Israel as his child and shows care, guidance, and discipline like a parent (Exodus 4:22; Deuteronomy 1:31; Hosea 11:1; see also Romans 9:4; Galatians 4:5; Ephesians 1:5).
Abba: This word was used within close family relationships and expressed deep trust and affection (similar to "dear father"). Jesus used this word when speaking to God (Mark 14:36). Through Jesus, all who become children of God are given the same privilege of addressing God in this personal way.