Tertullian – Treatise on the Soul (9.34, referencing 1 Cor. 7:14)

Hence in no case (I mean of the heathen, of course) is there any nativity which is pure of idolatrous superstition. It was from this circumstance that the apostle said, that when either of the parents was sanctified, the children were holy; and this as much by the prerogative of the (Christian) seed as by the discipline of the institution (by baptism, and Christian education). “Else,” says he, “were the children unclean” by birth: as if he meant us to understand that the children of believers were designed for holiness, and thereby for salvation; in order that he might by the pledge of such a hope give his support to matrimony, which he had determined to maintain in its integrity. Besides, he had certainly not forgotten what the Lord had so definitively stated: “Except a man be born of water and of the Spirit, he cannot enter into the kingdom of God;” in other words, he cannot be holy.

Tertullian. (1885). A Treatise on the Soul. In A. Roberts, J. Donaldson, & A. C. Coxe (Eds.), P. Holmes (Trans.), Latin Christianity: Its Founder, Tertullian (Vol. 3, pp. 219–220). Buffalo, NY: Christian Literature Company.