21. On this "spirit of adoption," that is on what the apostle means when he says "whereby we cry, Abba, Father," I have spoken at some length in my sermon on the text Galatians 4, 6, where the same words are used. In short, Paul describes here the power of the kingdom of Christ, the real work and the true exalted worship the Holy Spirit effects in believers: the comfort by which the heart is freed from the terror and fear of sin and given peace, and the heartfelt supplication which in faith expects of God an answer and his help. These blessings cannot be secured through the Law or our own holiness. By such means man could never obtain the comfort of God's grace and love to him; he would always remain in fear and dread of wrath and condemnation, and, because of such doubt, would flee from God, not daring to call upon him. But where there is faith in Christ, there the Holy Spirit brings the comfort spoken of, and a childlike trust which does not doubt that God is gracious and will answer prayer, because he has promised all these—grace and help, comfort, and answer to prayer—not for the sake of our worthiness, but for the sake of the name and merit of Christ, his Son.
22. Of these two works of the Holy Spirit, comfort and supplication, the prophet Zechariah (ch. 12, 10) said that God would establish a new dispensation in the kingdom of Christ when he should pour out "the spirit of grace and of supplication." The spirit he speaks of is the same who assures us that we are God's children, and desires us to cry to him with heartfelt supplications.
23. The Hebrew word "Abba"—which, as the apostle himself interprets it, means "Father"—is the word which the tiny heir lisps in childlike confidence to its father, calling him "Ab, Ab"; for it is the easiest word the child can learn to speak: or, as the old German language has it, almost easier still, "Etha, Etha." Such simple, childlike words faith uses toward God through the Holy Spirit, but they proceed out of the depth of the heart and, as afterwards stated, "with groanings which cannot be uttered." Rom 8, 26. Especially is this the case when the doubtings of the flesh and the terrors and torments of the devil bring conflict and distress. Man must defend himself against these and cries out: O dear Father! Thou art, indeed, my Father, for thou hast given thine only and beloved Son for me. Thou wilt not be angry with me or disown me. Or: Thou seest my distress and my weakness; do thou help and save me.
"The Spirit himself beareth witness with our spirit, that we are children of God."
24. That we are children of God and may confidently regard ourselves as such, we do not learn from ourselves nor from the Law. We learn it from the witness of the Spirit, who, in spite of the Law and of our unworthiness, testifies to it in our weakness and assures us of it. This witness is the experience within ourselves of the power of the Holy Spirit working through the Word, and the knowledge that our experience accords with the Word and the preaching of the Gospel. For thou art surely aware whether or no, when thou art in fear and distress, thou dost obtain comfort from the Gospel, and art able to overcome thy doubts and terror; to so overcome that thy heart is assured of God's graciousness, and thou no longer fleest from him, but canst cheerfully call upon him in faith, expecting help. Where such a faith exists, consciousness of help must follow. So Saint Paul says, Rom 5, 4-5: "Stedfastness worketh approvedness; and approvedness, hope: and hope putteth not to shame."
25. This is the true inward witness by which thou mayest perceive that the Holy Spirit is at work in thee. In addition to this, thou hast also external witnesses and signs: for instance, it is a witness of the Holy Spirit in thee that he gives thee special gifts, acute spiritual understanding, grace and success in thy calling; that thou hast pleasure and delight in God's Word, confessing it before the world at the peril of life and limb; that thou hatest and resistest ungodliness and sin. Those who have not the Holy Spirit are neither willing nor able to do these things. It is true, that even in the Christian, these things are accomplished in great weakness; but the Holy Spirit governs them in their weakness, and strengthens in them this witness, as Paul says again: "The Spirit also helpeth our infirmity." Rom 8, 26.