Augustine of Hippo — as excerpted in the Catena Aurea on Mark 9:43-50

“(de Con. Evan. 4. 6) Here truly it appears that they who do acts of devotedness in the name of Christ, even before they have joined themselves to the company of Christians, and have been washed in the Christian Sacraments, are more useful than those who though already bearing the name of Christians, by their doctrine drag their followers with themselves into everlasting punishment; whom also under the name of members of the body, He orders, as an offending eye or hand, to be torn from the body, that is, from the fellowship itself of unity, that we may rather come to everlasting life without them, than with them go into hell. But the separation of those who separate themselves from them consists in the very circumstance of their not yielding to them, when they would persuade them to evil, that is, offend them. If indeed their wickedness becomes known to all the good men, with whom they are connected, they are altogether cut off from all fellowship, and even from partaking in the heavenly Sacraments. If however they are thus known only to the smaller number, whilst their wickedness is unknown to the generality, they are to he tolerated in such a way that we should not consent to join in their iniquity, and that the communion of the good should not be deserted on their account.”