Martin Luther described a connection between the water of baptism and the blood Jesus shed on the cross. Why, he asks, should we see the blood of Christ in Baptism? His answer:
Because this Holy Baptism was purchased for us through this same blood, which he shed for us and with which he paid for sin.
The power of the cross is in Baptism
This blood and its merit and power he put into Baptism, in order that in Baptism we might receive it. For whenever a person receives Baptism in faith, this is the same as if he were visibly washed and cleansed of sin with the blood of Christ.
We do not attain the forgiveness of sins through our work, but rather through the death and the shedding of the blood of the Son of God. But he takes this forgiveness of sin and tucks it into Baptism.
This is what St. John was looking to when he mingled water and blood together, for, after all, it has in it that which was gained through the blood [I John 5:6]. And thus St. John deems the person who is baptized as having been washed in the blood of Christ.
[Christ's] blood is not that of a sinful man or the blood of a dead goat or ox; it is innocent, just, and holy. It is a blood of life. Therefore it also contains such strong salt and soap that, wherever it touches sin and uncleanness, it bites and washes it all away, eats and destroys both sin and death in an instant.
With the water and the blood
Thus St. John pictures our dear baptism for us in this way, so that we shall not regard and look only at the clear water, for, he says, Christ comes “not with water only” (as the Anabaptists blaspheme, saying it is nothing but water) “but with the water and the blood” [I John 5:6].
Through such words, he desires to admonish us to see with spiritual eyes and see in Baptism the beautiful, rosy-red blood of Christ, which flowed and poured from his holy side [John 19:34-35]. And therefore he calls those who have been baptized none other than those who have been bathed and cleansed in this same rosy-red blood of Christ.