“Look at it this way. If someone has a hundred sheep and one of them wanders off, doesn’t he leave the ninety-nine and go after the one? And if he finds it, doesn’t he make far more over it than over the ninety-nine who stay put? Your Father in heaven feels the same way. He doesn’t want to lose even one of these simple believers.
“If a fellow believer hurts you, go and tell him—work it out between the two of you. If he listens, you’ve made a friend. If he won’t listen, take one or two others along so that the presence of witnesses will keep things honest, and try again. If he still won’t listen, tell the church. If he won’t listen to the church, you’ll have to start over from scratch, confront him with the need for repentance, and offer again God’s forgiving love.
“Take this most seriously: A yes on earth is yes in heaven; a no on earth is no in heaven. What you say to one another is eternal. I mean this. When two of you get together on anything at all on earth and make a prayer of it, my Father in heaven goes into action. And when two or three of you are together because of me, you can be sure that I’ll be there.”
For I’ve seen some, when correcting their friends, clothe their bitterness or fury by claiming now zeal, now liberty. And since they follow impulse instead of reason, nothing good comes of it—they only make things worse. But between friends there is no excuse for this. A friend ought to sympathize with a friend, ought to condescend, ought to think of a friend’s fault as his or her own, correcting humbly and compassionately. You should look and sound the part, and don’t be afraid to weep if that’s appropriate; your friend needs to know the correction comes from love, not spite. If he or she rejects your rebuke, give a second chance. For your part, pray and weep; you may look troubled, but be sure to keep a holy affection towards your friend…In short, correcting and being corrected are normal, even proper, for friendship.
–Aelred of Rievaulx (1110-1167)
Spiritual Friendship 3.107-108.