When you touch a piece of iron that is lying in the forge and glowing, you touch not mere iron but fire which burns. Even if you see no fire but only iron (just as it is not seen to glow by day as much as by night), it is nevertheless not only iron but both iron and fire. In fact, it is so permeated by fire that nothing is felt or detected but pure fire. In the same way, Baptism should be viewed as incorporated into God’s name and completely permeated by it so that it is one essence, and thus has become a thing far different from other water.
Martin Luther on Holy Baptism: Sermons to the People 1525-39 (St. Louis: Concordia Publishing House, 2018), p. 29.
Something similar goes for the bread and wine (“This is my body,” “This is my blood”), which is one reason, and a good enough reason all by itself, to practice closed communion.