Is conversion something that occurs at once, in a crisis, or does it consist of a slowly continuing process?
Different answers must be given to this question. For regeneration, it was impossible to speak of a continuation or repetition. By the nature of the case, someone can only be born once. On the other hand, the consciousness of man goes up and down – is now darker, then clearer, subject to fluctuations and changes. It lies on the surface, on the outside of life and, consequently, can be seized by the still-continually-impure nature so that, in a certain sense it, again, turns away from God. Thus, the possibility exists for a continuing and repeated conversion. The Christian needs to die daily in repentance and [be] made alive in faith. The old man must be crucified anew again and again so that the new man can arise with all the more power.
Still, it remains true that, in its specific sense, conversion is something that occurs once and can only occur once and that, consequently, it stands out as a crisis in the lives of sinners. Thus, the old theologians had every right to bring conversion into the closest connection with regeneration and to regard conversion as the reverse side of the latter. Only where the consciousness is wholly fallen and turned away from God can it experience this great reversal, to which some give the designation of “first conversion.” Afterward, the contrast between the old and the new can no longer appear with the sharpness that it has in the first days of the activity of the new life. And, at a later point, it is tempered by the consciousness of justification that can never be entirely lost where it has once occurred. Although one must thus agree that the distance between first conversion and continuing conversion is not the same as the absolute difference between regeneration and the development of life, there still remains sufficient place for a distinction.
From: Reformed Dogmatics: Volume Four: Soteriology – The Application of the Merits of the Mediator by the Holy Spirit by Geerhardus Vos; translation from the Dutch supervised and edited by Richard B. Gaffin, Jr. (Bellingham, WA: Lexham Press, 2015), pp. 64-65.