A writer said:
"In the Western world we think of adoption in terms of taking a child from one family and making it a member of another. However, the Greek or Roman father adopted as a son his own child. Birth made him a child (teknon); adoption made him a son (huios). Between the period of birth and adoption, there were stages of growth, education and discipline, until the maturity was reached for adoption into sonship. With adoption the son was recognized as one who could faithful (sic) represent the father. He had arrived at the point of maturity, where the father could entrust him with the responsibility of overseeing the family business. The son becomes the “heir” of his father’s inheritance. Birth gives one the right to the inheritance, but adoption gives one the participation in the inheritance.
R. B. Jones, Bible commentator states: “To be a son is infinitely more than to be a child, and the terms are never loosely used by the Holy Spirit. It is not a difference in relationship, but in position. Every “born again” child of God has in him the nature of His Father, and is a beloved member of His Father’s family. Adoption cannot make the child any nearer or dearer, yet it gives the child a status he did not enjoy before, a position he did not occupy. It is his recognition as an adult son, the attaining of his maturity, the seal upon his growth to maturity of mind and character. A child is one born of God; a son is one taught of God. A child has God’s nature; a son has God’s character.
Another aspect of this Greek word, huios, that cannot be overlooked involves “likeness.” The New Testament contains the concept expressed in the proverb, “Like father, like son” (Matt. 5:45,48). It was typical Hebrew usage to employ the word “son” to express likeness. For instance, those who are peacemakers will be called God’s sons because they are like God (Matt. 5:9). God’s likeness, His image, will be “stamped” upon those who have been brought to maturity and adopted as sons (Rom. 8:29; I John 3:2-3).
Sonship and Maturity
The Bible speaks of sonship both in terms of “positional” and “experiential” truth. For example, some passages relate to the “positional” aspect of sonship, where God declares us legally to be “adopted” as sons through His sovereign election (Rom. 8:15; Gal. 3:26; 4:5-7). The Scriptures do clearly indicate that there is a degree to which we are expected to enter into sonship “experientially,” in this present age ( Matt. 5:9, 45; Rom. 8:14). For instance, we are exhorted in Hebrews 6:1 to “press on to maturity” (i.e.; “sonship”), to think as mature men (I Cor. 14:20), and we are to grow up in all aspects into Him (Ephes. 4:15).
All of these passages, and numerous others, call us to maturity, which is synonymous with the concept of sonship.” (Understanding “Sonship”
by Don Walker)
Another writer says:
"The word translated poorly as “adoption” is huiothesia and it occurs only five times in the New Testament. It is not found in the gospels although the proper meaning or principle is there. Before we examine the five Scriptures, and the context in which they are used, it is better to first look at the word huiothesia itself. Lexicons do not agree precisely on the meaning of the word. Typically, they give meanings such as, adoption as a son, but this is a vague compromise."
"The word huiothesia is never used to mean make anyone a son. It is to place a son. Each son who is placed already exists as a son. The Greek does not suggest making anyone a son and some lexicons point this out. Strong G5206 also gives the placing of a son. Following this up in Thayer we find: “That relationship which God was pleased to establish between himself and the Israelites, in preference to all other nations … that blessed state looked for in the future life after the visible return of Christ from heaven …”
"The word appears in five verses where we should read placing of a son rather than “adoption.”
"Rom 8:15 “For ye have not received the spirit of bondage again to fear; but ye have received the Spirit of adoption (placing of a son), whereby we cry, Abba, father”.
It is this indwelling spirit which enables those who are begotten from above to cry [krazo] “Abba Father”. Dr. Bullinger’s comments: Abba that is, father. Is said that slaves were never allowed to use the word Abba. Strictly therefore, it can be employed only by those who have received the gift of the Divine nature.
v16 The Spirit beareth witness with our spirit, that we are the children of God.
Rom 8:22,23 “For we know that the whole creation (ktisis) groaneth and travaileth in pain together until now, and not only they, but ourselves also, which have the firstfruits of the Spirit, even we ourselves groan within ourselves, waiting for the adoption (placing as sons), to wit, the redemption of our body”.
In this verse we can see an explanation of what adoption is, namely the redemption of our body.
See here for citation.
"The AV does not discriminate between teknon and huios." (W.E. Vine in Vine's New Testament Words)
(All emphasis are mine - SG)