As God by His almighty power engenders faith in man, Eph. 1, 19; John 6, 29, so also He works in the believer sanctification as the fruit of faith, 1 Thess. 5, 23. 24; 1 Cor. 3, 16; 6, 19; Eph. 2, 10. Nevertheless there is this distinction between conversion and sanctification, that in the former man is purely passive (pure passive se habet), while in the latter he cooperates with the Holy Ghost (active se habet sive cooperatur).
However, this cooperation must be rightly understood. It is not coordinate with the operation of the Holy Ghost, but subordinate to it. In other words, man cooperates in sanctification dependenter a Deo; that is to say, he works because and inasmuch as the Holy Ghost works in him, Rom. 8, 14: ὅσοι πνεύματι θεοῦ ἄγονται; Gal. 5, 16-18: εἰ δὲ πνεύματι ἄγεσθε. Hence every new spiritual impulse which the believer has, and every new good work which he does, is prompted and executed in him through the gracious power of the Holy Spirit, Phil. 1, 6; 2, 13.
The Formula of Concord writes of this very correctly (Thor. Decl., II, 65): "As soon as the Holy Ghost, ... through the Word and the holy Sacraments, has begun in us this His work of regeneration and renewal, it is certain that through the power of the Holy Ghost we can and should cooperate, although still in great weakness. But this . . . does not occur from our carnal, natural powers, but from the new powers and gifts which the Holy Ghost has begun in us in conversion, as St. Paul expressly exhorts that as workers together with Him we receive not the grace of God in vain, 2 Cor. 6, 1. But this is to be understood in no other way than that the converted man does good to such an extent and as long as God by His Holy Spirit rules, guides, and leads him and that, as soon as God would withdraw His gracious hand from him, he could not for a moment persevere in obedience to God. But if this were understood thus, ... that the converted man cooperates with the Holy Ghost in the manner as when two horses together draw a wagon, this could in no way be conceded without prejudice to the divine truth."
Hence not only conversion, but also sanctification depends entirely upon God's grace, 2 Cor. 5, 17. 18; 3, 5: ἡ ἱκανότης ἐκ τοῦ θεοῦ. This great truth, so clearly revealed in Scripture, should prompt the believer continually to perfect sanctification, Rom. 6, 14; 2 Cor. 7, 1; Heb. 12, 1. 2.