Luke 17:2-3

It would be better for him if a millstone [as large as one turned by a donkey] were hung around his neck and he were hurled into the sea, than for him to cause one of these little ones to stumble [in sin and lose faith]. [Matt 18:6; Mark 9:42] Pay attention and always be on guard [looking out for one another]! If your brother sins and disregards God’s precepts, solemnly warn him; and if he repents and changes, forgive him.