From the Word
14 For all who are led by the Spirit of God are sons of God. 15 For you did not receive the spirit of slavery to fall back into fear, but you have received the spirit of sonship. When we cry, “Abba! Father!” 16 it is the Spirit himself bearing witness with our spirit that we are children of God,
Romans 8:14–16, RSV
Paul describes here the power of the kingdom of Christ, the real work and the true exalted worship which the Holy Spirit effects in believers, the comfort by which the heart is freed from the fear and terror of sin and given peace, and the heartfelt supplication which in faith expects of God an answer and brings his help. These blessings cannot be secured through the law or through our own holiness. That we are children of God and may confidently regard ourselves as such, we learn from the witness of the Spirit, who, in spite of the law and our unworthiness, testifies to it in our weakness and assures us of it. Where there is faith in Christ, there the Holy Spirit brings comfort and childlike trust, which doubts not that God is gracious and will answer prayer as he has promised, not for the sake of our worthiness, but for the sake of the name and merit of Christ, his Son. 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, whether 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.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. It is a witness of the Holy Spirit in thee that he gives the special gifts, 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. Even in the Christian these things are accomplished in great weakness; but the Holy Spirit governs them in this weakness, and strengthens them in this witness, as Paul says: “The Spirit also helpeth our infirmities.”
Luther, Martin, and John Sander. Devotional Readings from Luther’s Works for Every Day of the Year. Augustana Book Concern, 1915, pp. 346–47.