Why did God choose the washing with water as the outward sign of joining sinners to himself and making them part of His Church? Answer, It signifies that we should continually drown our old sinful desires.
We must know what Baptism signifies and why God has ordained just this outward sign and ceremony for the Sacrament by which we are first received into the Christian Church. The act or ceremony is this: we are sunk under the water, which passes over us, and afterward are drawn out again. These two parts, (1) to be sunk under the water and (b) drawn out again, signify Baptism's power and work. It is nothing other than putting to death the old Adam and affecting the new man's resurrection after that [Romans 6:4-6]. Both of these things must take place in us all our lives. So a truly Christian life is nothing other than a daily Baptism, once begun and ever to be continued. For this must be done without ceasing, that we always keep purging away whatever belongs to the old Adam. Then what belongs to the new man may come forth. But what is the old man? It is what is born in human beings from Adam: anger, hate, envy, unchastity, stinginess, laziness, arrogance - yes, unbelief. The old man is infected with all vices and has by nature nothing good in him [Romans 7:18]. Now, when we have come into Christ's kingdom [John 3:5], these things must daily decrease. The longer we live the more we become gentle, patient, meek, and ever turn away from unbelief, greed, hatred, envy, and arrogance.
This is Baptisms true use among Christians, as signified by baptizing with water. (paragraphs 64-68)
Condensed from CONCORDIA: THE LUTHERAN CONFESSIONS, copyright 2005,2006 by Concordia Publishing House. Used by permission. All rights reserved. To purchase a copy of CONCORDIA, call 800-325-3040.