I. The Nature and Free Grace of Adoption
"Adoption is an act of God's free grace, whereby we are received into the number, and have a right to all the privileges of the sons of God." "God doth adopt children, when he taketh them which are strangers, and by nature children of wrath, into his family, and receiveth them into the number, and giveth them a right to all the privileges of the sons and daughters of God." "There is neither beauty, nor any lovely qualification, nor anything in the least, to move and incline God to adopt any whom he doth adopt, but it is an act only of his free grace and love." [Thomas Vincent - The Shorter Catechism]
"It is the granting of the right to eternal life, or the adoption as children, by Paul immediately named alongside the redemption from the law, Gal. 4:5..." [Herman Bavinck - Reformed Dogmatics]
"God is a Father by election, having chosen a certain number to be his children, upon whom he will entail heaven." "He showed power in being our Maker—but mercy in being our Father. When we were enemies, and our hearts stood out as garrisons against God—he conquered our stubbornness, and made us his children!" "Every adopted child may say, 'Even so, Father, for so it seemed good in your sight.'" [Thomas Watson - The Lord's Prayer]
II. The Christological Ground: Union with the Natural Son
"It was necessary that the Mediator should have that which it became him to confer upon us. It became him now, to confer upon us the right and title of the sons of God, which we had forfeited; that is, it became him to bring it to pass, that God might, for his Son's sake, adopt us as his children. This, however, Christ alone was able to effect, because he alone had the right thereof."
"Therefore, the Word, who is the natural Son of God, is alone our Mediator, in whom, as in the first begotten, we are adopted as the sons of God..." "They are adopted sons who do not begin at one and the same time to be, and to be sons; but who were already before they were adopted, or who had an existence before their adoption as sons. They have been made sons by law and the will of him who has adopted them, and given them the right and title of sons, so that they occupy the same place as if they were natural sons. So Adam, after his fall, and all those who are regenerated, are the adopted sons of God, received into favor with him on account of his natural Son, Jesus Christ. All these were the children of wrath before they were adopted into the family and church of Christ." [Zacharias Ursinus - Commentary on the Heidelberg]
"Who would have the presumption to arrogate to himself the honour of a son of God were we not gratuitously adopted as his sons in Christ? He being the true Son, has been given to us as a brother, so that that which he possesses as his own by nature becomes ours by adoption, if we embrace this great mercy with firm faith."
"We must, however, constantly hold, that God never was a Father to angels and men save in respect of his only-begotten Son: that men, especially, who by their iniquity were rendered hateful to God, are sons by gratuitous adoption, because he is a Son by nature."
"Christ, then, is the mirror in which we ought, and in which, without deception, we may contemplate our election. For since it is into his body that the Father has decreed to ingraft those whom from eternity he wished to be his, that he may regard as sons all whom he acknowledges to be his members, if we are in communion with Christ, we have proof sufficiently clear and strong that we are written in the Book of Life." [John Calvin - Institutes]
"This adoption is a legal act, whereby God places the sinner in the status of a child, but does not change him inwardly any more than parents by the mere act of adoption change the inner life of an adopted child. The change that is effected concerns the relation in which man stands to God." "Just as the believers on the ground of Christ’s righteousness receive the forgiveness of sins, so are they also adopted as children, υἱοι θεου (not τεκνα). This υἱοθεσια, which thus rests on a declaration of God, has been acquired by Christ, Gal. 4:5, and becomes our portion through faith, 3:26." "The believers are thereby placed in the same status as Christ, who is the firstborn among many brethren, Rom. 8:29. He was the Son of God by nature, 8:32, and was so declared at his resurrection, 1:3; the believers become ὑιοι θεου by adoption." [Herman Bavinck - Reformed Dogmatics]
III. The Connection to Faith, Regeneration, and the Spirit of Sonship
"Are all the children of men the adopted children of God? No; only such persons are adopted as do believe in Christ." [Thomas Vincent - The Shorter Catechism]
"It is now proper to consider the nature of this faith, by means of which, those who are adopted into the family of God obtain possession of the heavenly kingdom." [John Calvin - Institutes]
"God is a Father by special grace. He consecrates the elect by his Spirit—and infuses a supernatural principle of holiness, therefore they are said to be 'born of God.'" "We may know God is our Father by RESEMBLING him. The child is his father's picture." "Whoever God adopts for his child is like him; he not only bears his heavenly Father's name—but his image." [Thomas Watson - The Lord's Prayer]
"...and so by that same Spirit assured of this their sonship, ib. Gal. 4:6." [Herman Bavinck - Reformed Dogmatics]
IV. The Glorious Privileges and Fatherly Discipline
"The privileges which the adopted children of God have a right unto are— 1. God's fatherly protection of them from temporal and spiritual evils... 2. God's fatherly provision of all needful things, both for their soul and body... 3. God's fatherly correction of them... 4. God's audience and return to their prayers... 5. A sure title to the inheritance of the kingdom of heaven." [Thomas Vincent - The Shorter Catechism]
"God being a Father, if he frowns, if he dips his pen in gall, and writes bitter things, if he DISCIPLINES—it is all done in love! A father loves his child as well when he chastises and disciplines him—as when he settles his inheritance on him." "If God is our Father—he will TEACH us. What father will refuse to counsel his son? Does God command parents to instruct their children, and will not he instruct his children?" "If God is our Father—he will be full of SYMPATHY towards his children." "God makes his children equal in honor to the angels." "God has made his children, by adoption, nearer to himself than the angels. The angels are the friends of Christ: believers are his members—and this honor have all the saints."
V. The Indissoluble Security of the Eternal Inheritance
"If God is our Father—he will not disinherit us. He may for a time desert his children—but will not disinherit them." "It is evident that God's children cannot be finally disinherited, by virtue of the eternal decree of heaven." "God's decree is the very pillar and basis on which the saints' perseverance depends. That decree ties the knot of adoption so fast, that neither sin, death, nor hell, can break it asunder." [Thomas Watson - The Lord's Prayer]
"As sons they are then at the same time κατ’ ἐπαγγελίαν κληρονόμοι, Gal. 3:29, 4:7, Rom. 8:17; and because this inheritance still lies in the future, the υἱοθεσία in its full realization is also still an object of hope, Rom, 8:23." "But even if the 'Rechtsfolge der Adoption' still awaits, the believers have nevertheless already on earth been adopted as children; they are sealed by the Holy Spirit as guarantee and pledge unto the day of their redemption, 2 Cor. 1:22, 5:5, Ef." [Herman Bavinck - Reformed Dogmatics]