A term borrowed from childhood's language to express filial address to God (Mark 14:36; Rom. 8:15; Gal. 4:6). The corresponding Hebrew word is Ab; it is common in compound proper names in the forms Ab and Abi, as Abimelech, Abner or Abiner, Eliab.
Note Paul's words: "For you did not receive the spirit of bondage again to fear, but you received the Spirit of adoption [sonship] by which we cry out, 'Abba, [that is] Father'" (Rom. 8: 15) . We are to become very sons of our Father! You and as many as are willing of all who have ever lived are to become, first, begotten sons of God the Father, and then, finally, born as literal sons of God in the God Family (I Cor. 15:23, I John 3:1-3).
God begins the birth of His sons very small, just as we begin the birth of our own children. "Now he that hath wrought us for the self same thing is God, who also hath given unto us the EARNEST of the Spirit" (II Cor. 5:5). An "earnest" is a very small down payment carrying with it a promise of more to come, until the full commitment is reached.
"For as many as are led by the Spirit of God , they are the sons of God" (Rom. 8: 14). There are no capital letters in the Greek, by the way. It is only the translators who decided to capitalize "Son" when it referred to Jesus and not capitalize "son" when it referred to us. Most reverent and sincere humility no doubt, but it leads to misunderstanding.
"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" (Ro m. 8: 15). That word "adoption" can just as well be translated "sonship"- or even " Sonship" - but according to the very scriptures the translators were translating, it never entered their minds that we could be the actual "Sons of God. " They only took words like " brethren," "son," "Father" to be metaphors, niceties, condescension from God as it were. But this scripture clearly declares that we human beings, after receiving God 's Holy Spirit, are to call God Abba, which is Aramaic for " father", and then amplifies and reinforces that by adding " Father" from the Greek. An actual Father, not a pretend Father.
Perhaps it was fear of the same charge of "blasphemy" leveled against Jesus by the religious people of His day that caused those men to use the translation "adoption" in the clear light of the context of a double-language (Aramaic, Greek ) " Father-son" relationship discussed in Romans eight?
"The Spirit itself beareth witness with our spirit, that we are the children of God " (verse 16). This Spirit of God is, remember, the only source of this humanly inconceivable concept of what it is that God has in store for those who love Him. Now these "children of God" discussed in Romans are not born yet. They have just begun, just been spiritually conceived. They are not changed yet from physical to spiritual totally; they just have the "earnest" of that yet future total change.