Augustine (354-430)… Baptism, he said, is nothing else than salvation itself (A Treatise on the Merits and Forgiveness of Sins 1:34) it “brings salvation” (Letter 98, To Boniface 1). We are “saved by baptism”; “the salvation of man is effected by baptism” (Against Two Letters of the Pelagians, 1:55). The “apostolic tradition” teaches the “inherent principle, that without baptism… it is impossible for any man to attain to salvation and everlasting life” (ibid, 1:34).
Thomas Aquinas represents medieval Catholic thinking. He declared that “no one can obtain salvation but through Christ… But for this end is baptism conferred on a man, that being regenerated thereby, he may be incorporated in Christ… Consequently, it is manifest that all are bound to be baptized: and that without Baptism there is no salvation for men (Summa Theologica, 68:1).