In his book Luther and Aquinas on Salvation (1965), the Roman Catholic theologian Stephen Pfurgner nails Luther’s views about what creates certainty in the Christian:
“To this notion of “grace” there corresponds also the manner in which I become certain of it. For certainty does not come to me from any kind of reflection on myself or on my state. On the contrary, it comes solely through hearing the Word, solely and because and in so far as I cling to the Word of God and its promise. Certainty of grace for the believer therefore does not arise from a feeling of confidence; it is not psychological, as Catholic critics have often represented it. Faith only as acceptance of the Word, effective of salvation, is for Luther the decisive source of certainty. Not indeed that subjective experience is to be excluded: the experience of comfort can be incorporated in the certainty of salvation. But God can withdraw feeling, at any rate for a time, without the confidence of faith being thereby dissolved. A sense of comfort therefore is in no way the real basis for the certainty of salvation: this is the Word of God and the promise it includes.” (pp. 125 and 126)
To demonstrate this, he quotes Luther saying :
If you have received forgiveness of sins, do not on that account be secure (secures). You are just, holy, from outside yourself (extrinsece). It is through mercy and compassion that you are just. It is not my disposition or a quality of my heart, but something outside myself – the divine mercy – which assures us that our sins are forgiven” (WE 40 I, pp. 588f, in Pfurgner, 124, 125)
He also quotes Luther on how he teaches the certainty of grace or salvation:
[“We must daily more and more strive to get out of uncertainty into certainty and occupy ourselves with destroying at its root that utterly pernicious error”] (that man cannot know whether or not he is in a state of grace), by which the whole world is seduced. If we doubt God’s grace and do not believe that God is well-pleased in us for Christ’s sake, then we are denying that Christ has redeemed us – indeed, we question outright all his benefits. (WE 40 I, p. 579, 17f, in Pfurgner, 37, and 120)
I would add here: holding to good and salutary thoughts like Luther’s here are not necessary for one’s personal salvation – but they are necessary!
Pfurgner also provides several other fine quotes on the topic from Luther’s Galatians commentary:
“Our ground is the following: The Gospel teaches us not to look to our good deeds and perfections, but to the God of promise, to Christ the Mediator himself. The Pope on the other hand orders us not to look to the God of the promise, not to Christ the high-priest, but to our works and merits. On that side there follow necessarily doubt and despair, but on this certainty and joy of spirit since I cling to God who cannot lie….” (WE 40 I, pp. 588f, in Pfurgner, 37)
Also this one where Luther emphasizes the “to me”:
“But do not pass over contemptuously the pronoun “nostris”, for it will avail thee nothing to believe that Christ offered himself for the sins of the other saints and to doubt in regard to thy own. For the godless and the devils also believe that. Much rather must thou accept with constant trust the fact that it holds also for thine and that thou art one of those for whose sins he was offered. This faith justifies thee and makes Christ dwell, live and rule in thee.” (WE 40 I, p. 458, 20f, in Pfurgner, 37 and 38).
Pfurgner sums things up this way:
“Luther’s interpretation of Catholic teaching maintains therefore: the Roman Church (‘the Pope’) does not recognize the certainty of salvation. It abandons the individual to doubt and despair. For it bases justification on the works of men, on self-sanctification. But by his own merits no man can become completely just before God. It follows that he must remain in distress and turmoil of conscience.” (38)