Tertullian, On Baptism 13

Those most wicked characters raise these questions. They claim baptism isn't necessary for people who have faith, since Abraham pleased God through the sacrament of faith without any water at all. But in everything, the later revelation settles the matter, and what comes after overrides what came before. Salvation used to be possible through bare faith before the Lord's passion and resurrection. 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 law. The law of baptism was imposed, and a 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 was appended the definition: "Unless someone is born of water and the Spirit, he cannot enter the kingdom of God" (John 3:5), which bound faith to the necessity of baptism. From then on, all who believed were baptized. Even Paul, the moment he believed, was baptized. This is exactly what the Lord commanded in that stroke of blindness: "Get up and go into Damascus; there you will be told what you must do" (Acts 22:10)—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.