Daily death and resurrection
As Luther said so well, the Christian life consists entirely in the daily return to baptism: “a truly Christian life is nothing else than a daily baptism.” The Large Catechism, one of Luther’s most carefully prepared works, explains this classic Lutheran description of the Christian life:
These two parts, to be sunk under the water and drawn out again, signify the power and operation of Baptism, which is nothing else than putting to death the old Adam, and after that the resurrection of the new man, both of which must take place in us all our lives, so that a truly Christian life is nothing else than a daily baptism, once begun and ever to be continued. For this must be practised without ceasing, that we ever keep purging away whatever is of the old Adam, and that that which belongs to the new man come forth. But what is the old man? It is that which is born in us from Adam, angry, hateful, envious, unchaste, stingy, lazy, haughty, yea, unbelieving, infected with all vices, and having by nature nothing good in it. Now, when we are come into the kingdom of Christ, these things must daily decrease, that the longer we live we become more gentle, more patient, more meek, and ever withdraw more and more from unbelief, avarice, hatred, envy, haughtiness.
This daily baptism is not merely symbolic but rather an actual death and resurrection, as witnessed by Article V of the Apology of the Augsburg Confession:
. . . there is first terror and anxiety in the conscience. Thus contrition and faith go side by side. One is putting off the body of sins; the other is the rising again through faith. Neither ought these words, mortification, quickening, putting off the body of sins, rising again, to be understood in a Platonic way, concerning a feigned change; but mortification signifies true terrors, such as those of the dying, which nature could not sustain unless it were supported by faith. So he names that as the putting off of the body of sins which we ordinarily call contrition, because in these griefs the natural concupiscence is purged away. And quickening ought not to be understood as a Platonic fancy, but as consolation which truly sustains life that is escaping in contrition. Here, therefore, are two parts: contrition and faith. For as conscience cannot be pacified except by faith, therefore faith alone quickens, according to the declaration, Hab. 2:4; Rom. 1:17: The just shall live by faith.
Luther’s Large Catechism continues:
This is the true use of Baptism among Christians, as signified by baptizing with water. Where this, therefore, is not practised, but the old man is left unbridled, so as to continually become stronger, that is not using Baptism, but striving against Baptism. For those who are without Christ cannot but daily become worse, according to the proverb which expresses the truth, “Worse and worse–the longer, the worse.” If a year ago one was proud and avaricious, then he is much prouder and more avaricious this year, so that the vice grows and increases with him from his youth up. A young child has no special vice; but when it grows up, it becomes unchaste and impure, and when it reaches maturity, real vices begin to prevail the longer, the more. Therefore the old man goes unrestrained in his nature if he is not checked and suppressed by the power of Baptism. On the other hand, where men have become Christians, he daily decreases until he finally perishes. That is truly to be buried in Baptism, and daily to come forth again. Therefore the external sign is appointed not only for a powerful effect, but also for a signification. Where, therefore, faith flourishes with its fruits, there it has no empty signification, but the work [of mortifying the flesh] accompanies it; but where faith is wanting, it remains a mere unfruitful sign. And here you see that Baptism, both in its power and signification, comprehends also the third Sacrament, which has been called repentance, as it is really nothing else than Baptism. For what else is repentance but an earnest attack upon the old man [that his lusts be restrained] and entering upon a new life? Therefore, if you live in repentance, you walk in Baptism, which not only signifies such a new life, but also produces, begins, and exercises it. For therein are given grace, the Spirit, and power to suppress the old man, so that the new man may come forth and become strong.
In summary, “Ethical progress is only possible by returning to Baptism” (Oswald Bayer, Living by Faith, Chapter 5, Kindle location 779 of 1207). This spiritual growth or “ethical progress” is received only by faith in the gospel. Baptism and the other means of grace restore God’s image by communicating the gospel.