What then is the part assigned to faith in justification? According to
Luther (and
Calvin also), the faith that justifies is not, as the Catholic Church teaches, a firm belief in
God’s revealed truths and promises (
fides theoretica, dogmatica), but is the infallible conviction (
fides fiducialis, fiducia) that
God for the sake of Christ will no longer impute to us our sins, but will consider and treat us, as if we were really just and holy, although in our inner selves we remain the same sinners as before. Cf. Solid. Declar. III, sec. 15: “Through the obedience of Christ by faith the just are so declared and reputed, although by reason of their corrupt nature they still are and remain, sinners as long as they bear this mortal body.” This so-called “fiduciary faith” is not a religious-moral preparation of the soul for sanctifying grace, nor a free act of cooperation on the part of the sinner; it is merely a means or spiritual instrument (
instrumentum, organon leptikon) granted by
God to assist the sinner in laying hold of the righteousness of
God, thereby to cover his sins in a purely external manner as with a mantle.
newadvent.org/cathen/08573a.htm