Augustine of Hippo
Catena Aurea · Matthäusevangelium 18
Moreover if thy brother shall trespass against thee, go and tell him his fault between thee and him alone: if he shall hear thee, thou hast gained thy brother. But if he will not hear thee, then take with thee one or two more, that in the mouth of two or three witnesses every word may be established. And if he shall neglect to hear them, tell it unto the Church: but if he neglect to hear the Church, let him be unto thee as an heathen man and a Publican.
(Serm. 82, 7.) That is, regard him no longer in the number of thy brethren. Though even thus we are not to neglect his salvation; for the heathens themselves, that is, the gentiles and pagans, we do not indeed regard in the number of our brethren, yet we ever seek their salvation.