Let it be noted, too, that if the object of justifying faith is the entire Scriptures, there can be no assurance of the forgiveness of sins. Again and again doubt will rise in one’s mind, and indeed in the mind of the most learned theologian, as to whether his understanding of all Scripture, including the historical portions of it, is the correct one. The inevitable product of Bellarmine’s contention, as indeed of the whole Roman Catholic system, is the monstrum incertitudinis [monstrous uncertainty]. A person may have the fides iustificans [faith that justifies] and salvans [saves] though he is ignorant of certain parts of Scripture and even in weakness errs in certain doctrines of Scripture. The Bible teaches this explicitly (Rom. 14:1 ff.). Orthodoxy and true faith are not identical. Genuine orthodoxy is in every case the result of saving faith (for only in those hearts which accept the Gospel the Holy Ghost is active), but there are cases where saving faith has not as yet produced the acceptance of all doctrines of Scripture. This truth has been unhesitatingly professed by Luther, the Lutheran Confessions, and the Lutheran dogmaticians, though, at the same time, they unhesitatingly have refused to concede to any man the right to surrender any portion of the Christian doctrine.
-- Pieper, Francis. Christian Dogmatics, Vol. II. Saint Louis (Concordia Publishing House): 1953. (424-425)