St. Augustine on Baptismal Regeneration

It is this one Spirit who makes it possible for an infant to be regenerated . . . when that infant is brought to baptism; and it is through this one Spirit that the infant so presented is reborn. For it is not written, ‘Unless a man be born again by the will of his parents’ or ‘by the faith of those presenting him or ministering to him,’ but, ‘Unless a man be born again of water and the Holy Spirit.’ The water, therefore, manifesting exteriorly the sacrament of grace, and the Spirit effecting interiorly the benefit of grace, both regenerate in one Christ that man who was generated in Adam.

Letters 98:2 [A.D. 412] – St. Augustine

Gregory of Nazianz on Baptismal Regeneration

Such is the grace and power of baptism; not an overwhelming of the world as of old, but a purification of the sins of each individual, and a complete cleansing from all the bruises and stains of sin. And since we are double-made, I mean of body and soul, and the one part is visible, the other invisible, so the cleansing also is twofold, by water and the Spirit; the one received visibly in the body, the other concurring with it invisibly and apart from the body; the one typical, the other real and cleansing the depths.

Oration on Holy Baptism 7–8 [A.D. 388] – Gregory of Nazianz