A pastor pours water on the head of a child and baptizes him in the name of the Father, the Son and the Holy Spirit. If this has not been done to a person as a child, it can be done to the person as an adult. Why?
　Jesus commanded to baptize. He said that through baptism a person becomes His disciple. When we are baptized, we get the great gifts Jesus Christ earned for us on the cross. Jesus has provided us the right to be children of God. Through baptism we are given forgiveness, salvation and the eternal life as a gift. The Bible says that we are clothed in Christ when we are baptized. From then on we find refuge and safety in Jesus and His work on the cross.

It’s natural to wonder how just pouring some water can do so much. God has promised to act when a person is baptized. He has promised to take a person as His child, to forgive and to save. God has chosen such a way to deliver us what Jesus has earned. There is surely a great wisdom in that God works exactly this way.

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.
(Matthew 28:19-20, ESV)

Go deeper with The Small Catechism - Holy Baptism