There is a difference between the expressions "children of God" and "sons of God."
Adoption in the general usage of the word, means that one who is not a member of a family by birth or relationship, is given, by adoption, a legal standing in that family, and is thus counted as a family member. In the Bible the word means a different thing. It means son-placing, and shows us the truth that all who are born anew into the family of God become at the same time "the sons of God," with the privileges and responsibilities which go with that position.
The word "adoption" occurs in the following passages: "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" (Rom. 8:15). "And not only they, but ourselves also, which have the firstfruits of the Spirit, even we ourselves groan within ourselves, waiting for the adoption, to wit, the redemption of our body" (Rom. 8:23). "Who are Israelites; to whom pertaineth the adoption, and the glory, and the covenants, and the giving of the law, and the service of God, and the promises" (Rom. 9:4). "To redeem them that were under the law, that we might receive the adoption of sons" (Gal. 4:5). "Having predestinated us unto the adoption of children by Jesus Christ to Himself, according to the good pleasure of His will" (Eph. 1:5).
We have already mentioned that adoption means son-placing. As far as relationship is concerned, we become God's children by the new birth. As to our position in His family, we become sons by adoption. Two different words are used to distinguish between our relationship as children and our position as sons. The Greek noun teknon means a born-one, and has reference to our coming into God's family by the new birth; while the word huios means son, and refers to our standing and position before God. Thus teknon is properly translated child (pl. "children"), and huios is properly rendered "son" (pl. "sons"), though this distinction is not always maintained in our Authorized Version. For example, the word tekna is used in John 1:12, but it is translated "sons"; whereas in Hebrews 12:5 the noun huiois is translated "children."
When one receives the Lord Jesus Christ as Saviour, he is at that moment both born into the family of God and adopted into the family of God. The two things take place simultaneously, at the very moment of believing. By birth, we become God's children; by adoption, we become His full-grown sons. While it is true that we need to "grow in grace, and in the knowledge of our Lord and Saviour Jesus Christ" (II Pet. 3:18), we are nevertheless regarded and treated as full-grown sons from the very moment that we receive Christ and are thus adopted into God's family. From that moment on, we have the privileges and responsibilities that go with our position as sons.
Notice the following references to our adoption: "Even as Abraham believed God, and it was accounted to him for righteousness. Know ye therefore that they which are of faith, the same are the children [sons] of Abraham" (Gal. 3:6, 7). Here we see that adoption comes just as the new birth comes, by simple faith in the Lord Jesus Christ as Saviour. "For ye are all the children [sons] of God by faith in Christ Jesus" (Gal. 3:26).
"But when the fulness of the time was come, God sent forth His Son, made of a woman, made under the law, 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. Wherefore thou art no more a servant, but a son; and if a son, then an heir of God through Christ" (Gal. 4:4-7). We see here the truth that our present position of sons in God's family is different from the case of believing Jews in Old Testament days who, as far as their position was concerned, were servants and not sons.
"But ye, brethren, are not in darkness, that that day [the day of the Lord] should overtake you as a thief. Ye are all the children [sons] of light, and the children [sons] of the day: we are not of the night, nor of darkness. Therefore let us not sleep, as do others; but let us watch and be sober" (I Thess. 5:4-6). Position carries privilege, but it always carries responsibility also. As sons of God we are enjoined to "watch and be sober" in our Christian lives, especially in view of the Lord's coming.
In order to teach us to "watch and be sober," to be diligent in our Christian lives, our faithful Father chastens His sons and daughters. "And ye have forgotten the exhortation which speaketh unto you as unto children [sons], My son, despise not thou the chastening of the Lord, nor faint when thou art rebuked of Him: for whom the Lord loveth He chasteneth, and scourgeth every son whom He receiveth. If ye endure chastening, God dealeth with you as with sons; for what son is he whom the father chasteneth not? But if ye be without chastisement, whereof all are partakers, then are ye bastards, and not sons" (Heb. 12:5-8).
Our Lord Jesus Christ, by His death and resurrection, has given all believers a perfect position before God, the standing as sons, "according as He hath chosen us in Him before the foundation of the world, that we should be holy and without blame before Him in love: having predestinated us unto the adoption of children [sons] by Jesus Christ to Himself, according to the good pleasure of His will, to the praise of the glory of His grace, wherein He hath made us accepted in the Beloved" (Eph. 1:4-6). All this is by His grace, and for His glory. "But we see Jesus, who was made a little lower than the angels for the suffering of death, crowned with glory and honour; that He by the grace of God should taste death for every man. For it became Him, for whom are all things, and by whom are all things, in bringing many sons unto glory, to make the captain of their salvation perfect through sufferings. For both He that sanctifieth and they who are sanctified are all of one: for which cause He is not ashamed to call them brethren" (Heb. 2:9-11).
It is by adoption, then, that we have our position before God. And it is because of our adoption that the Holy Spirit has been given us; "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" (Rom 8:15); "and because ye are sons, God hath sent forth the Spirit of His Son into your hearts, crying, Abba, Father" (Gal. 4:6). The Holy Spirit of adoption, who dwells in the hearts of all the Lord's own, witnesses to our new birth and our resulting relationship to the Father as His children: "The Spirit [Himself] beareth witness with our spirit, that we are the children of God" (Rom. 8:16). He also bears witness to our heirship: "And if children, then heirs; heirs of God, and joint-heirs with Christ; if so be that we suffer with Him, that we may be also glorified together" (vs. 17).
The Spirit of adoption also witnesses to our nearness to the Father. We "have received the Spirit of adoption, whereby we cry, Abba, Father" (Rom. 8:15). "And because ye are sons, God hath sent forth the Spirit of His Son into your hearts, crying, Abba, Father" (Gal. 4:6). The word "Abba" is a word of tender affection for "Father," so used by our Lord Himself in Mark 14:36. The latest born child of God, who so short a time ago feared God, may now take upon his lips this blessed word which assures him that he is now a son in the family. It is both remarkable and touching to notice how the newly-saved Christian immediately recognizes that God is his Father, and delights so to address Him in perfect liberty.
For the Spirit of adoption is the Spirit of liberty. "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" (Rom. 8:15), "and where the Spirit of the Lord is, there is liberty" (II Cor. 3:17).
Though God has given us the son-place and we are now adopted into His family, we do not as yet have all the benefits of this adoption. We are "waiting for the adoption, to wit the redemption of our body" (Rom. 8:23). This will take place, of course, when the Lord Jesus comes for His own "For our conversation [citizenship] is in heaven: from whence also we look for the Saviour, the Lord Jesus Christ: who shall change our vile body, that it may be fashioned like unto His glorious body, according to the working whereby He is able even to subdue all things unto Himself" (Phil. 3:20, 21). "Beloved, now are we the sons of God, and it doth not yet appear what we shall be: but we know that when He shall appear, we shall be like Him; for we shall see Him as He is" (I John 3:2). Every believer has been marked out for this glory, this final aspect of adoption. God has "predestinated us unto the adoption of children [sons] by Jesus Christ to Himself, according to the good pleasure His will" (Eph. 1:5). "For whom He did foreknow, He also did predestinate to be conformed to the image of His Son, that He might be the firstborn among many brethren (Rom. 8:29).
As has already been stated, we become children of God by spiritual birth, and sons of God by spiritual adoption. We are children in our relationship and sons in our position. When a father and son are partners in business, the firm is not known as "Brown and Child," but as "Brown and Son," for the word "son" carries in it the thought of responsibility and privilege.
That all believers are sons of God is seen in the fact that all Scriptures having to do with the Christian life are addressed to all. There are no distinctions. All true believers have the same privileges, the same position, and the same responsibilities. What God says to one of us, He says to all.
In summary, adoption gives us (1) position. We are full grown sons of God, with (2) the privilege of being His representatives in this world. What responsibility this carries with it, as we "serve the living and true God; and…wait for His Son from heaven" (I Thess. 1:9, 10)! And when He comes, we shall have (3) final perfection, for seeing Him we shall be made like Him, "and so shall we ever be with Lord" (I Thess. 4:17).
From Great Doctrines Relating to Salvation by John B. Marchbanks. Neptune, NJ: Loizeaux Brothers, 1970. Chapter 10.