They are extremely comforting for all those wrestling in prayer against the devil, the world, and your sinful flesh.
With these words, then, Paul wants to indicate the weakness there still is in the pious, as in Rom. 8:26: “The Spirit helps us in our weakness.” For because the awareness of the opposite is so strong in us, that is, because we are more aware of the wrath of God than of His favor toward us, therefore the Holy Spirit is sent into our hearts. He does not whisper and does not pray but cries very loudly: “Abba! Father!” and intercedes for us, in accordance with the will of God, with sighs too deep for words. How?
In deep terrors and conflicts of conscience we do indeed take hold of Christ and believe that He is our Savior. But then the Law terrifies us most, and sin disturbs us. In addition, the devil attacks us with all his stratagems and his fiery darts (Eph. 6:16), trying with all his might to snatch Christ away from us and to rob us of all comfort. Then there is nothing to keep us from succumbing and despairing, for then we are the bruised reed and the dimly burning wick (Is. 42:3). Meanwhile, however, the Holy Spirit is helping us in our weakness and interceding for us with sighs too deep for words (Rom. 8:26), and He is bearing witness with our spirit that we are children of God (Rom. 8:16). Thus the mind is strengthened amid these terrors; it sighs to its Savior and High Priest, Jesus Christ; it overcomes the weakness of the flesh, regains its comfort, and says: “Abba! Father!” This sighing, of which we are hardly aware, Paul calls a cry and a sigh too deep for words—a sigh that fills heaven and earth. He also calls it a cry and a sigh of the Spirit, because when we are weak and tempted, then the Spirit sets up this cry in our heart.
No matter how great and terrible the cries are that the Law, sin, and the devil let loose against us, even though they seem to fill heaven and earth and to overcome the sighs of our hearts completely, still they cannot do us any harm. For the more these enemies press in upon us, accusing and vexing us with their cries, the more do we, sighing, take hold of Christ; with heart and lips we call upon Him, cling to Him, and believe that He was born under the Law for us, in order that He might redeem us from the curse of the Law and destroy sin and death. When we have taken hold of Christ by faith this way, we cry through Him: “Abba! Father!” And this cry of ours far exceeds the cry of the devil.
But we are far from supposing that this sigh which we emit amid the terrors and in our weakness is a cry—so far indeed that we hardly understand that it is even a sigh. For so far as our own awareness is concerned, this faith of ours, which sighs to Christ in temptation, is very weak. That is why we do not hear this cry. We have only the Word. If we take hold of this in the struggle, we breathe a little and sigh. To some extent we are aware of this sigh, but we do not hear the cry. But “He who searches the hearts of men,” Paul says (Rom. 8:27), “knows what is the mind of the Spirit.” To Him who searches the hearts this sigh, which seems so meager to the flesh, is a loud cry and a sigh too deep for words, in comparison with which the great and horrible roars of the Law, sin, death, the devil, and hell are nothing at all and are inaudible. It is not without purpose, then, that Paul calls this sigh of the pious and afflicted heart the crying and indescribable sighing of the Spirit; for it fills all of heaven and earth and cries so loudly that the angels suppose that they cannot hear anything except this cry.
Within ourselves, however, there is the very opposite feeling. This faint sigh of ours does not seem to penetrate the clouds in such a way that it is the only thing to be heard by God and the angels in heaven. In fact, we suppose, especially as long as the trial continues, that the devil is roaring at us terribly, that heaven is bellowing, that the earth is quaking, that everything is about to collapse, that all the creatures are threatening us with evil, and that hell is opening up in order to swallow us. This feeling is in our hearts; we do not hear these terrible voices or see this frightening face. And this is what Paul says in 2 Cor. 12:9: that the power of Christ is made perfect in our weakness. For then Christ is truly almighty, and then He truly reigns and triumphs in us when we are, so to speak, so “all-weak” that we can scarcely emit a groan. But Paul says that in the ears of God this sigh is a mighty cry that fills all of heaven and earth.
Likewise in Luke 18:1–8, in the parable of the unjust judge, Christ calls this sigh of the pious heart a cry, and a cry that cries to God incessantly day and night. He says: “Hear what the unrighteous judge says. And will not God vindicate His elect, who cry to Him day and night? Will He delay long over them? I tell you, He will vindicate them speedily.” Today, amid all the persecution and opposition from the pope, the tyrants, and the fanatical spirits, who attack us from the right and from the left, we cannot do anything but emit such sighs. But these have been our cannon and our instruments of war; with them we have frustrated the plans of our opponents all these years, and we have begun to demolish the kingdom of Antichrist. But they will provoke Christ to hasten the day of His glorious coming, when He will abolish all principalities, powers, and might, and will put all His enemies under His feet. Amen.
Thus in Exodus the Lord says to Moses at the Red Sea (14:15): “Why do you cry to Me?” That was the last thing Moses was doing. He was in extreme anguish; therefore he was trembling and at the point of despair. Not faith but unbelief appeared to be ruling in him. For Israel was so hemmed in by the mountains, by the army of the Egyptians, and by the sea that it could not escape anywhere. Moses did not even dare mumble here. How, then, did he cry? Therefore we must not judge according to the feeling of our heart; we must judge according to the Word of God, which teaches that the Holy Spirit is granted to the afflicted, the terrified, and the despairing in such a way that He encourages and comforts them, so that they do not succumb in their trials and other evils but conquer them, though not without very great fear and effort.
The papists imagined that the saints had the Holy Spirit in such a way that they never experienced or had any temptations. They speak about the Holy Spirit only speculatively, as the fanatical spirits do today. But Paul says that the power of Christ is made perfect in our weakness (2 Cor. 12:9), and that the Holy Spirit helps us in our weakness and intercedes for us with sighs too deep for words (Rom. 8:26). Therefore we have the greatest need for the aid and comfort of the Holy Spirit, and He is also nearest to us when we are at our weakest and nearest to despair. If someone passes through evil with a courageous and happy spirit, then the Holy Spirit has already performed His work in him. But He really performs His work in those who are thoroughly terrified and who have come near to what the psalm calls “the gates of death” (9:13). Thus I have just said that Moses saw the very presence of death in the water and wherever he turned his gaze. Therefore he was in the deepest anxiety and despair, and undoubtedly he sensed in his heart the loud cry of the devil against him, saying: “This entire people will perish today, for they cannot escape anywhere. You alone are responsible for this great calamity, for you led them out of Egypt.” Then there came the cry of the people, who said (Ex. 14:11–12): “Is it because there are no graves in Egypt that you have taken us away to die in the wilderness? It would have been better for us to serve the Egyptians than to die in the wilderness.” Then the Holy Spirit was present in Moses, not speculatively but actually; He interceded for him with sighs too deep for words, so that Moses sighed to God and said: “Lord, it was at Thy command that I led the people out. Therefore do Thou help!” This sigh is what He calls “crying.”
I have discussed this at some length in order to show what the work of the Holy Spirit is and how He usually carries it out. In temptation we must not on any account decide this matter on the basis of our feeling or of the cry of the Law, sin, and the devil. If we want to follow our feeling here or to believe those cries, we shall decide that we are bereft of all help from the Holy Spirit and that we have been utterly banished from the presence of God. Should we not rather remember, then, that Paul says that the Holy Spirit helps us in our weakness and cries: “Abba! Father!”? That is, He emits what seems to us to be some sort of sob and sigh of the heart; but in the sight of God this is a loud cry and a sigh too deep for words. In every temptation and weakness, therefore, just cling to Christ and sigh! He gives you the Holy Spirit, who cries: “Abba! Father!” Then the Father says: “I do not hear anything in the whole world except this single sigh, which is such a loud cry in My ears that it fills heaven and earth and drowns out all the cries of everything else.”
You will notice that Paul does not say that the Spirit intercedes for us in temptation with a long prayer, but that He intercedes with a sigh, and one that is too deep for words. He does not cry loudly and tearfully: “Have mercy on me, O God” (Ps. 51:1); but He merely utters the words of a cry and a sigh, which is “Oh, Father!” This is indeed a very short word, but it includes everything. Not the lips, but the feelings are speaking here, as though one were to say: “Even though I am surrounded by anxieties and seem to be deserted and banished from Thy presence, nevertheless I am a child of God on account of Christ; I am beloved on account of the Beloved.” Therefore the term “Father,” when spoken meaningfully in the heart, is an eloquence that Demosthenes, Cicero, and the most eloquent men there have ever been in the world cannot attain. For this is a matter that is expressed, not in words but in sighs, which are not articulated in all the words of all the orators; for they are too deep for words.
Luther, M. (1999). Luther’s works, vol. 26: Lectures on Galatians, 1535, Chapters 1-4. (J. J. Pelikan, H. C. Oswald, & H. T. Lehmann, Eds.) (Vol. 26, pp. 381–385). Saint Louis: Concordia Publishing House.