Romans 8:15 — 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.
spirit of bondage — a spirit possessed by and characteristic of slaves
adoption = son-placing. The place and state of a son given to one to whom it doesn not belong by natural descent.
In Galatians 4:6 (And because ye are sons, God hath sent forth the Spirit of his Son into your hearts, crying, Abba, Father.), the Spirit says this. Here is is said by those who are enabled by the Spirit.
Abba (Aramaic) — unreasoning trust (not the same as Father), filial confidence, communion, obedience — the love and intelligent trust of a child.
The adoption of children, as we speak of it in English today, refers to the taking in of other people's children. This is not the meaning of the Greek huiothesia, for according to Galatians 4:1-7 this "placing as sons" affected those already children. This is not to imply, of course, that a stranger could not also be taken in and given a place as a full-grown son, but the point is that "adoption" here does not refer to mere acceptance into the family, but to a declaration of full sonship, with all its rights and privileges — and responsibilities.
In the life of the Hebrew boy there came a time "appointed of the father," when "adoption" proceedings took place and the boy was declared to be the son and heir of the father.
Before that time he had been a son, indeed, but "under tutors and governors." He had been told what he must and must not, what he might and might not, do. In this he differed nothing from a servant.
But finally the "time appointed" arrives! He is growing up now. It is assumed that he will no longer need overseers to keep him in check. There has come to be a natural understanding and co-operation between father and son. And so the "adoption" proceeding take place: a declaration, public and official, that the lad now enters into all the rights and privileges of full sonship.
Such is the meaning of the word adoption as used in the writings of Paul. How all this opens up the meaning of Romans 8:15. True, we Gentile believers were once strangers and aliens, graciously taken into the family of God, but a careful examination of the above and related passages on "adoption" will clearly reveal that more than present-day adoption is meant. — C. R. Stam, The Epistle of Paul to the Romans, page 193-194.