“When I am converted by the Gospel, the Holy Spirit is present. He takes me as clay and makes of me a new creature, which is endowed with a different mind, heart, and thoughts, that is, with a true knowledge of God and a sincere trust in His grace.
To summarize, the very essence of my heart is renewed and changed. This makes me a new plant, one that is grafted on Christ the Vine and grows from Him. My holiness, righteousness, and purity do not stem from me, nor do they depend on me.
They come solely from Christ and are based only in Him, in whom I am rooted by faith, just as the sap flows from the stalk into the branches. Now I am like Him and of His kind. Both He and I are of one nature and essence, and I bear fruit in Him and through Him. This fruit is not mine; it is the Vine’s.
Thus Christ and the Christians become one loaf and one body, so that the Christian can bear good fruit—not Adam’s or his own, but Christ’s. For when a Christian baptizes, preaches, consoles, exhorts, works, and suffers, he does not do this as a man descended from Adam; it is Christ who does this in him.
The lips and tongue with which he proclaims and confesses God’s Word are not his; they are Christ’s lips and tongue. The hands with which he toils and serves his neighbor are the hands and members of Christ, who, as He says here, is in him; and he is in Christ.
Behold, with the words ‘He who abides in Me, and I in him’ (John 15:5) Christ wants to indicate that Christianity is not brought in from without; it is not put on like a garment, nor does it consist in the adoption of a new manner of living, which, like monasticism and self-chosen sanctity, is concerned with works.
It is a new birth brought about by God’s Word and Spirit; there must be an entirely new man from the bottom of his heart. Then, however, when the heart is born anew in Christ, fruits will follow naturally, such as the confession of the Gospel, works of love, obedience, and patience.”
–Martin Luther, Luther’s Works, Vol. 24: Sermons on the Gospel of St. John: Chapters 14-16, ed. Jaroslav Jan Pelikan, Hilton C. Oswald and Helmut T. Lehmann (Saint Louis: Concordia Publishing House, 1999), John 15:5.