Tertullian, On Baptism 13

So those utterly wicked characters are stirring up these questions. They claim baptism isn't necessary for people who have faith, since Abraham pleased God through the sacrament of faith alone, without any water at all. But they're arguing backward: in every case, what comes later decides the matter, and the subsequent overrides the prior. Before the Lord's passion and resurrection, salvation was possible through bare faith. But once faith expanded to include belief in his birth, passion, and resurrection, the sacrament was amplified — baptism was added as a seal, a kind of garment for faith that had previously stood naked and lacked power without its own governing law. That law of baptizing was imposed, and the form was prescribed: "Go, make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit" (Matt 28:19). To that law this definition was appended: "Unless someone is born of water and Spirit, he cannot enter the kingdom of heaven" (John 3:5) — which bound faith to the necessity of baptism. From that point on, every believer was baptized. Even Paul, the moment he believed, was baptized. That's exactly what the Lord commanded in that stroke of blindness: "Get up, go into Damascus, and there you will be told what you must do" — namely, be baptized, the one thing he still lacked. He had already learned and believed thoroughly that the Nazarene is the Lord, the Son of God.