Simply put, Baptism is the application of water to a person in the name of the Holy Trinity—the Father and the Son and the Holy Spirit. Plain water is truly used in this Sacrament, but it is not plain water alone.
Our Lord Jesus Christ, following His death and resurrection and just before His ascension, gave the Church the gift of Holy Baptism:
“Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to observe all that I have commanded you. And behold, I am with you always, to the end of the age” (Matthew 28:19-20
So it is that the water of Holy Baptism is used according to Jesus’ command and combined with the name of God and all that name stands for. This God-given reality changes everything about this particular application of water.
What Does Baptism Do?
In his Small Catechism, Martin Luther provided a concise explanation for us:
It works forgiveness of sins, rescues from death and the devil, and gives eternal salvation to all who believe this, as the words and promises of God declare. (SC, Baptism, Second Part)
To learn more about what Lutherans believe and teach about Holy Baptism, check out the book Lutheranism 101.