From Romans Commentary (1519)
Paul speaks of the “Spirit of adoption” to show in what way we become God’s children, and to exalt the grace of God. We are not children of God by nature (as Christ alone is), nor by descent, nor because of our merits, but only because of our gracious adoption by God as children in Christ…
“The Spirit of adoption by which we cry Abba Father,” (Rom. 8:15). Paul means to say, you have been freed from fear and have received the Spirit of adoption by which you trust in God. This trust he shows very clearly by the words, “by which we cry Abba Father.” This is the cry of the heart which is full of childlike trust and knows not fear. That the cry is not one of the mouth by of the heart is clear from Gal. 4:6-7, “Because you are sons, God has sent forth the Spirit of his Son into your hearts crying, Abba Father. Therefore you are no more a servant, but a son; and if a son then an heir of God through Christ.”
“The Spirit bears witness with our spirit, that we are the children of God” (Rom. 8:16)… This witness of the Spirit is the filial trust of our heart in God. St. Bernard of Clairvaux in the first chapter of his sermon for the Feast of the Annunciation to Blessed Mary comments on this verse, “I believe that this witness consists of three things: first believe that you can have forgiveness of sins only through the gracious favor of God; second, do not call a second work your own unless God has given it to you; and finally, believe that you cannot earn eternal salvation by good works, for this salvation was given to you out of pure grace”… The witness of the Holy Spirit must be regarded as the beginning and, so to speak, as the foundation of faith.
Martin Luther (1483-1546) was a German priest and theologian, a seminal figure of the Protestant Reformation. His teaching about justification by faith, revealed in his study of the Pauline Epistles, became the core of Protestant teaching about salvation, and inspired a wide-reaching series of reform in Christian ministry, worship, and spiritual practice. Martin Luther is commemorated on February 18 on the calendars of several Lutheran and Anglican Churches. This translation of the text is by Theodore Mueller (Grand Rapids, MI: Zondervan, 1954).