In the discussion of the subject of conversion the question whether conversion is successive (conversio succession) or instantaneous (conversio momentanea) has been given much consideration. Since conversion takes place through the kindling of faith in the heart by the Holy Ghost, it is clear that it occurs in a moment (conversio momentanea), namely, in that moment when the Holy Spirit through the means of grace engenders faith in the contrite sinner. Hence, as soon as the penitent sinner possesses the first spark or longing of faith, he is already fully converted. (Conversio temporis momento fit, . . . veluti ἔν ῥιπῇ ὄμματος. Calov.)
Conversion may be said to be successive (conversio succession) only in case certain acts (actus praeparatorii) which commonly precede it are regarded as a part of conversion. To these actus praeparatorii belong the inculcation of the divine Law, the conviction of the sinner of his guilt and condemnation, the incitement of the terrores conscientiae, and the like. Properly speaking, however, these acts of the Holy Ghost only prepare the sinner for conversion, but do not convert him; for conversion, properly speaking, occurs only in that moment when the Holy Spirit through the Gospel changes the alarmed and despairing sinner into a rejoicing believer in Christ.
For this reason we must not speak of a middle state (status medius) between conversion and non-conversion (homo renascens, homo in statu medio constitutus), since this is both unscriptural and synergistic. It is unscriptural; for Holy Scripture recognizes only two classes of men, the converted and the unconverted, or what is the same, believers and unbelievers, John 3, 18. 36; Mark 16, 16; 1 Pet. 2, 25. According to Scripture it is impossible for a person to be in a middle state even for a moment, for there is no middle ground between belief and unbelief, between life and death, Luke 11, 23.
Theologians who in opposition to Scripture reject the instantaneous character of conversion and explain it as a long-drawn-out process,1 during which the sinner is first enlightened, then awakened, and finally brought to the decision to accept Christ, commonly do so in the interest of synergism, that is to say, to support their erroneous views that the awakened sinner in the final analysis must convert himself with spiritual powers bestowed upon him by the Holy Ghost (Latermann).
As a matter of fact, the objections of modern rationalistic theologians to the instantaneous character of conversion are really not directed against the conversio momentanea, but against the sola gratia; for synergistic rationalism regards conversion both as an act of divine grace and as an act of human meritorious effort. It champions the doctrine of “successive conversion” and of “the middle state” since according to its erroneous view God endows the sinner only with the potentiality, or ability, to believe and not with faith itself. Faith, it is claimed, is man’s own free, conscious, and deliberate self-determination (Selhstbestimmung), accomplished through spiritual powers granted to him by God. From this it is clear that the onslaught upon the Scriptural doctrine of instantaneous conversion is, in the last analysis, directed against divine monergism in conversion, or against the sola gratia.
It goes without saying that what is here said of synergism is true also of Arminianism. Both insist upon successive conversion because both hold that man in the last instance must convert himself. Hominis voluntas in conversions non est otiosa, sed agit aliquid. Against this error the Formula of Concord testifies (Thor. Decl., II, 62) : “No modus agendi, or no way whatever of working something good in spiritual things, can be ascribed to man before his conversion.”
See also Publishers comment in subchapter Scriptural basis for the doctrine (15.1), and subchapter Reiterated conversion (15.11)
Turn to me, for I will deliver you. (...)
This conversion does not take place in one day or in one month, as the Lent was under the papacy, but it is a perpetual duty of the people of God.
Quotes from Luther's works, St.L, vol.6, page 542-543, section 27, Isaiah 44:22.