Monergism

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.
Westminster Shorter Catecism Q 34

“Extol and magnify God’s mercy, who has adopted you into his family; who, of slaves, has made you sons; of heirs of hell, heirs of the promise. Adoption is a free gift. He gave them power, or dignity, to become the sons of God. As a thread of silver runs through a whole piece of work, so free grace runs through the whole privilege of adoption. Adoption is greater mercy than Adam had in paradise; he was a son by creation, but here is a further sonship by adoption. To make us thankful, consider, in civil adoption there is some worth and excellence in the person to be adopted; but there was no worth in us, neither beauty, nor parentage, nor virtue; nothing in us to move God to bestow the prerogative of sonship upon us. We have enough in us to move God to correct us, but nothing to move him to adopt us, therefore exalt free grace; begin the work of angels here; bless him with your praises who has blessed you in making you his sons and daughters.”
Thomas Watson, A Body of Divinity, p. 240