The Holy Spirit's work in assuring believers of their adopted-son standing before God. Romans 8:15-17: For ye have not received the spirit of bondage again to fear; but ye have received the Spirit of adoption, whereby we cry, Abba, Father. The Spirit itself beareth witness with our spirit, that we are the children of God: And if children, then heirs; heirs of God, and joint-heirs with Christ. Galatians 4:5-7 develops the parallel: God sent His Son to redeem them that were under the law, that we might receive the adoption of sons. And because ye are sons, God hath sent forth the Spirit of his Son into your hearts, crying, Abba, Father. The Spirit's adoption-witness is not external sentiment but inward testimony — a settled conviction the Spirit produces in the believer's heart that he is, in fact, God's adopted child. Abba is the Aramaic intimate address (Father, Papa) that Christ Himself used in Gethsemane (Mark 14:36), now placed in the believer's mouth by the same Spirit.