23 For it is the Church that gives birth to all, either within her pale, of her own womb; or beyond it, of the seed of her bridegroom—(either of herself, or of her handmaid.) But Esau, even though born of the lawful wife, was separated from the people of God because he quarrelled with his brother. And Asher, born indeed by the authority of a wife, but yet of a handmaid, was admitted to the land of promise on account of his brotherly good-will. Whence also it was not the being born of a handmaid, but his quarrelling with his brother, that stood in the way of Ishmael, to cause his separation from the people of God; and he received no benefit from the power of the wife, whose son he rather was, inasmuch as it was in virtue of her conjugal rights that he was both conceived in and born of the womb of the handmaid. Just as with the Donatists it is by the right of the Church, which exists in baptism, that whosoever is born receives his birth; but if they agree with their brethren, through the unity of peace they come to the land of promise, not to be again cast out from the bosom of their true mother, but to be acknowledged in the seed of their father; but if they persevere in discord, they will belong to the line of Ishmael. For Ishmael was first, and then Isaac; and Esau was the elder, Jacob the younger. Not that heresy gives birth before the Church, or that the Church herself gives birth first to those who are carnal or animal, and afterwards to those who are spiritual; but because, in the actual lot of our mortality, in which we are born of the seed of Adam, "that was not first which is spiritual, but that which is natural, and afterward that which is spiritual." But from mere animal sensation, because "the natural man receives not the things of the Spirit of God," arise all dissensions and schisms. And the apostle says that all who persevere in this animal sensation belong to the old covenant. that is, to the desire of earthly promises, which are indeed the type of the spiritual; but "the natural man receives not the things of the Spirit of God."
24. At whatever time, therefore, men have begun to be of such a nature in this life, that, although they have partaken of such divine sacraments as were appointed for the dispensation under which they lived, they yet savor of carnal things, and hope for and desire carnal things from God, whether in this life or afterwards, they are yet carnal. But the Church, which is the people of God, is an ancient institution even in the pilgrimage of this life, having a carnal interest in some men, a spiritual interest in others. To the carnal belongs the old covenant, to the spiritual the new. But in the first days both were hidden, from Adam even to Moses. But by Moses the old covenant was made manifest, and in it was hidden the new covenant, because after a secret fashion it was typified. But so soon as the Lord came in the flesh, the new covenant was revealed; yet, though the sacraments of the old covenant passed away; the dispositions peculiar to it did not pass away. For they still exist in those whom the apostle declares to be already born indeed by the sacrament of the new covenant, but yet capable, as being natural, of receiving the things of the Spirit of God. For, as in the sacraments of the old covenant some persons were already spiritual, belonging secretly to the new covenant, which was then concealed, so now also in the sacrament of the new covenant, which has been by this time revealed, many live who are natural. And if they will not advance to receive the things of the Spirit of God, to which the discourse of the apostle urges them, they will still belong to the old covenant. But if they advance, even before they receive them, yet by their very advance and approach they belong to the new covenant; and if, before becoming spiritual, they are snatched away from this life, yet through the protection of the holiness of the sacrament they are reckoned in the land of the living, where the Lord is our hope and our portion. Nor can I find any truer interpretation of the scripture, "Your eyes saw my substance, yet being imperfect" considering what follows, "And in Your book shall all be written."
Source: On Baptism, Against the Donatists (New Advent)