Rom 8:15

8:15 Spirit of adoption. In addition to justification and freedom from condemnation (v. 1), believers are taken into the family of God and are inwardly persuaded by the Spirit that they belong there. The cry of the believer, “Abba! Father!” (the Aramaic word Abba was used by Jesus Himself for God, Mark 14:36) indicates how vividly union with Christ was realized in the experience of the New Testament church. The cry is an expression of an assured awareness of sonship. The idea of adoption does not appear in the Old Testament legal system, and Paul seems to have borrowed this apt concept from Roman law, filling it out with the biblical theology of God’s fatherhood over His people.