In Lutheranism we believe that God has specifically promised to act through certain means, what we call Means of Grace; these Means are
Word and Sacrament. A point we would look at is that in Scripture God consistently uses means to make Himself known, and to act, God always encounters people through mediation. For example in Scripture Moses petitions God to let him see God in His fullness, God responds "No man may see Me and live." And Moses only catches a glimpse of God's "backside" and this itself causes Moses' face to shine so brilliantly that it requires him to wear a veil to cover his face for a while afterward so that others aren't blinded by that light. So God acts through Means, or makes Himself known through things; Moses' first encounter with God is in the form of a burning bush, God leads the Israelites by pillars of fire and smoke, God heals the Israelites using a bronze serpent, God sends prophets to speak His Word, etc. And, of course, most importantly there is Jesus Christ, the Word made flesh, God made man.
So since we see that God uses means, it shouldn't be surprising that God has promised us means by which we can have certainty of where God is and what God is doing. So that by the preaching of the Gospel God is most certainly acting, acting to come down and grant us faith, as St. Paul writes, "Faith comes by hearing, and hearing by the Word of Christ." (
Romans 10:17) or earlier, "I am not ashamed of the Gospel, for it is the power of God to save all who believe." (
Romans 1:16). Likewise we see that God has instituted Sacraments, sacred mysteries whereby (if we were to quote St. Augustine) "the Word is attached to an element and it becomes a Sacrament, that is, visible Word"; so we see that God has attached certain promise, has attached His Word, to water in Baptism. The water itself isn't what matters, it is the Word which God has united to the water, namely "Baptism now saves you" (
1 Peter 3:21), "All of you who were baptized into Christ have put on Christ" (
Galatians 3:21), "All of us who have been baptized into Christ Jesus have been baptized into His death" (
Romans 6:3), "He cleansed her by the washing of water by the word" (
Ephesians 5:26), "He saved us, not because of works done by us in righteousness, but according to his own mercy, by the washing of regeneration and renewal of the Holy Spirit" (
Titus 3:5), "Repent and be baptized every one of you in the name of Jesus Christ for the forgiveness of your sins, and you will receive the gift of the Holy Spirit" (Acts 2:38), and so on and so forth. Therefore God has attached promises to this thing, Baptism, with Baptism it is
water with the Word, not just mere water. It is the Word that does all these things and makes all these things happen, as the Prophet Isaiah has written, "So shall My word be that goes out from My mouth; it shall not return to Me empty, but it shall accomplish that which I purpose, and shall succeed in the thing for which I sent it." (
Isaiah 55:11).
When God speaks, it happens. When God promises, it is certain. So when God speaks to us, in the waters of Baptism, that we are united to Christ in His death and resurrection, clothed with Christ, our sins forgiven, receiving the Holy Spirit, and indeed are saved we can be confident in our faith that these things are true for God has promised them to us.
Therefore it fundamentally isn't the water that matters at all; but what God has attached to the water--namely His Word, thereby water and Word is a Sacrament, specifically the Sacrament of Baptism. If it were only water, it would not be Baptism, but because it is water
with the Word it is Baptism.