Calvin says that we need something to assure us that God really loves us. We hear it, and that ought to be enough, but we being weak need more:
And what if someone asks whether the word is sufficient? True, it should indeed suffice, and we must blame our vices and our unbelief for the fact that we do not attribute enough weight and authority to God’s word. The fact is that we are so weak that even after God speaks to us, we need him to add some confirmation to bring us to the understanding of what he wants to show us. For example, we will be told daily that our Lord Jesus Christ yielded himself to death for us in testimony of the love which God bears us and by which he adopts us as his children. That information will be repeated often, and it is worthy of our trust and of our receiving it as teaching from heaven. But our natures are so weak that even if we are instructed in it every day, we cannot accept what God says to us, but are always doubtful. There is no steadfastness in us. Our Lord supplies our deficiency and adds baptism, by which he gives us a visible sign that we are washed by the blood of Jesus Christ.
Now I’m not sure I agree with how Calvin words this, as if the Sacraments would be unnecessary in a perfect world, however, I do agree with him as to what the sacraments communicate.
Baptism says, to the party baptized, that they are washed by the blood of Jesus Christ. Analyze it all you want, but whatever you decide, you must believe it.
You can read the entire sermon over at David’s blog.