Since conversion consists essentially in the bestowal of faith in Christ, it is obvious that the terminus a quo of conversion is unbelief, while its terminus ad quern is true confidence in Christ Jesus, Acts 26, 18: ἐπιστρέψαι ἀπο σκότους εἰς φῶς; 2 Cor. 3, 14—16. Quenstedt: Conversio prime est infidelium, ... et sic notat conversionem ab infidelitate ad fidem.
Only then is a sinner converted when in place of infidelity, which by nature is found in every human heart, 1 Cor. 2, 14, there is found in him faith in the gracious promises of God for Christ's sake (propter Christum). As long as a person is without faith in Christ, he is unregenerate, or unconverted, no matter whether in the sight of man he is a criminal or a saint, an illiterate or a sage. Upon all who are without Christ, Scripture pronounoes the verdict that they are without God in this world and have no hope, Eph. 2, 12.
However, as soon as a person believes in Christ, his conversion, or return to God, has been fully accomplished, even though his faith should be a mere spark (scintillula). Of all who believe in Christ, St. Paul writes: “But now in Christ Jesus ye who sometime were far off are made nigh by the blood of Christ,” Eph. 2, 13. According to these words it is faith in Christ that distinguishes those who are “far off,” or the unregenrate, from those who ar > “made nigh,” or the regenerate. In other words, as the apostle clearly teaches, conversion takes place through faith in the blood of Christ.
Unless this truth is constantly borne in mind, it is impossible to avoid the mistake of regarding the unconverted as converted, or vice versa, the converted as unconverted.
Properly speaking, the starting-point of conversion is unbelief, its terminus, saving faith in Christ, and its essential feature, the kindling of faith (donatio fidei). However, since unbelief is always joined with spiritual darkness, the dominion of Satan, idolatry, the state of sin, etc., also these factors may be said to constitute the starting-point of conversion. On the other hand, faith is always joined with spiritual life, communion with God, the keeping of the divine commandments, etc., and therefore also these things may be said to be the terminus of conversion. Thus Scripture itself speaks of conversion as a turning from darkness to light, from the power of the devil to God, Acts 26, 18, from idolatry to the worship of the living God, Acts 14, 15; 1 Thess. 1, 9, from transgression to the keeping of the divine Law, Ezek. 18, 21, etc.
In all these passages unbelief and faith are described according to their outward manifestation, or fruits, so that we may rightly say: All who are in spiritual darkness, or under the dominion of Satan, or in the power of sin, or in the thraldom of idolatry are unconverted, while those who have spiritual life, are in communion with God, and possess new spiritual powers to keep the commandments of God are truly converted. But it must not be forgotten that to be converted, in its proper and narrow sense, always means to come to faith in the Gospel of Christ, the Savior of sinners, Acts 11, 20. 21; 1 Pet. 2, 25, whereas spiritual life, communion with God, and the keeping of the divine commandments are, properly speaking, fruits, or effects, of conversion. A person truly performs the will of God only after his will has been inclined, or turned, to God through faith in Christ, in other words, after he has been converted,.