It is true, indeed, that Christ is perfectly and completely applied to the soul in the first act for righteousness. “Justification being a relative change, properly admits no degrees, but is perfected together, and at once, in one only act; though as to its manifestation, sense, and effects, it has various degrees,” says William Ames.
But the application of Christ to us, for wisdom and sanctification, is not perfected in one single act, but rises by many, and slow degrees to its just perfection. And though we are truly said to have come to Christ when we first believe (John 6:35), yet the soul after that is still coming to Him by further acts of faith: “To whom [coming] as unto a living stone” (1 Pet 2:4). The participle notes a continued motion, by which the soul gains ground, and still gets nearer and nearer to Christ; growing still more inwardly acquainted with Him.
The knowledge of Christ grows upon the soul as the morning light, from its first spring to the perfect day (Pro 4:18). Every grace of the Spirit grows, if not sensibly, yet really; for it is in discerning the growth of sanctification, as it is in discerning the growth of plants, which we perceive to have grown, rather than grow. And as it thrives in the soul, by deeper movements of the habits, and more promptitude and spirituality in the acting; so Christ, and the soul proportionally, close more and more inwardly and efficaciously, till at last it is wholly swallowed up in Christ’s full and perfect enjoyment.