New Defender's Study Bible Notes
8:15 adoption. We, as children of God by faith in Christ, have become “adopted” into the family of the Father. Jesus Christ Himself is still the only begotten of the Father, one with Him in divine essence from eternity, but we have the inestimable privilege of being adopted as sons and daughters by Him.
8:15 Abba. “Abba” is an Aramaic term of endearment for “Father.” Note also Mark 14:36. The combined use of the Aramaic and Greek might mean something like “dear Father,” or even “Papa.”