Few better explanations of the ancient Catholic understanding of baptism can be found than that of Tertullian in his work on baptism, namely De baptismo 1. Baptism in the Catholic regula fidei, imparted many things on the believer, such as it remitted the original sin, imparted the Holy Spirit and regenerated the heart, sanctified, set apart, adoption, made us partakers of the divine nature and so on. Tertullian says it well,
"...our sacrament of water, by which the sins of our earlier blindness are washed away and we are released for eternal life."
"All waters, therefore, by reason of the original sign at their beginning, are suitable, after God has been invoked, for the sacrament of sanctification. The Spirit immediately comes from heaven upon the waters, and rests upon them, making them holy of Himself; and having been thus sanctified they absorb at the same time the power of sanctifying."
And what is to be done immediately after the act of water baptism? Tertullian explained the ancient apostolic tradition,
"After coming from the place of washing we are thoroughly anointed with a blessed unction...the unction runs on the body but profits us spiritually, in the same way that Baptism is itself a corporal act by which we are plunged in the water, while its effect is spiritual, in that we are freed from sins. After this, the hand is imposed for a blessing, invoking and inviting the Holy Spirit." 2
1. Written during Tertullian's orthodox phase.
2. The quotes from Tertullian; (De baptismo. 1, 1; 4, 1-4; 7, 2; 8, 1). Cf. this same understanding of baptism in the patristic testimony; St. Clement of Rome 46, 6, Barnabas 11, 11, St. Justin Martyr (Apologia prima pro Christianis. 61), St. Theophilus of Antioch (Ad Autolycum. 2, 16), St. Irenaeus (Adversus haereses. 3, 17, 1), Clement of Alexandria (Paed. 1, 6, 26), to name but a small group of the Catholic fathers.