The Conferences Of John Cassian by John Cassian
Chapter V. The answer, how a lasting friendship can only exist among those who are perfect.
Joseph: For this reason we said that the full and perfect grace of friendship can only last among those who are perfect and of equal goodness, whose likemindedness and common purpose allows them either never, or at any rate hardly ever, to disagree, or to differ in those matters which concern their progress in the spiritual life. But if they begin to get hot with eager disputes, it is clear that they have never been at one in accordance with the rule which we gave above. But because no one can start from perfection except one who has begun from the very foundation, and your inquiring is not with regard to its greatness, but as to how you can attain to it, I think it well to explain to you, in a few words, the rule for it and the sort of path along which your steps should be directed, that you may be able more easily to secure the blessing of patience and peace.