“Naturally, God is the Almighty One, and He could, had He wanted to, have perfectly sanctified all His children in the moment of regeneration. But that apparently was not His will; in the recreation He does not deny Himself as Creator.
All the life of the creature is born, grows up, and only gradually reaches its maturity. Because the spiritual life is actually life it comes to be and it develops in this same way.
God does not inject the righteousness and holiness of Christ into us mechanically, or pour it out as one does water into a vessel, but He works it out in us in an organic way. Hence the one detail does not conflict with the other when the Scripture constantly presents the matter as though the believers must become that which they are.
The kingdom of heaven is a gift of God (Luke 12:32) and yet it is a treasure of great worth which must be sought after (Matt. 6:33 and 13:46). The believers are the branches of the vine, and they can, accordingly, do nothing without Christ, and yet they are told in His word to remain in Him, in His word, and in His love (John 15:1ff).
They were elected in Christ from before the foundation of the world, and yet they must be diligent to make their calling and their election sure (Eph. 1:4 and 2 Peter 1:10). They have been sanctified by the one sacrifice of Christ, and must nevertheless follow after sanctification, without which no man shall see the Lord (Heb. 10:10 and 12:14).
They are complete, and nevertheless require constant perfecting and establishment (Col. 2:10 and 1 Peter 5:10). They have put on the new man, and must nevertheless constantly put him on (Eph. 4:24 and Col. 3:10).
They have crucified the flesh with the affections and lusts, and must nevertheless still mortify their members which are upon the earth (Gal. 5:24 and Col. 3:5). It is God who works in them both to will and to do according to His good pleasure, and yet they must work out their salvation with fear and trembling (Phil. 2:12–13).
These data do not conflict with each other. The one is simply the ground and guarantee of the other. Because sanctification, like the whole of salvation, is the work of God, we are admonished, obliged, to a new obedience, and we are also qualified for it.
He grants abundant grace not that we should instantly or suddenly be holy and continue to rest in this holiness, but that we should persevere in the struggle and remain standing.
He hears our prayers but does it in accordance with the law and order which He has fixed for the spiritual life. Hence we are always of good courage, for He who has begun a good work in us will finish it until the day of Jesus Christ.
The believers can and they will become holy because in Christ they are holy.”
–Herman Bavinck, The Wonderful Works of God (trans. Henry Zylstra; Glenside, PA: Westminster Seminary Press, 1909/2019), 484-485.