26 Therefore, whether they seem to abide within, or are openly outside, whatsoever is flesh is flesh, and what is chaff is chaff, whether they persevere in remaining in their barrenness on the threshing-floor, or, when temptation befalls them, are carried out as it were by the blast of some wind. And even that man is always severed from the unity of the Church which is without spot or wrinkle, who associates with the congregation of the saints in carnal obstinacy. Yet we ought to despair of no man, whether he be one who shows himself to be of this nature within the pale of the Church, or whether he more openly opposes it from without. But the spiritual, or those who are steadily advancing with pious exertion towards this end, do not stray without the pale; since even when, by some perversity or necessity among men, they seem to be driven forth, they are more approved than if they had remained within, since they are in no degree roused to contend against the Church, but remain rooted in the strongest foundation of Christian charity on the solid rock of unity. For hereunto belongs what is said in the sacrifice of Abraham: "But the birds divided he not."
Source: On Baptism, Against the Donatists (New Advent)