I found this quote in the book “A Diary of Readings” compiled by John Baillie (1955, New York, Charles Scribner's Sons). This is an excerpt from Fénelon’s “Letters and Reflections” (English translation, 1906) which probably was written in the early 1700’s from Cambray.
This is a reading for day 60, with the heading of “The faults of Others.”
"Charity does not demand of us that we should not see the faults of others; we must in that case shut our eyes. But it commands us to avoid attending unnecessarily to them, and that we be not blind to the good, while we are so clear-sighted to the evil that exists. We must remember too God’s continual kindness to the most worthless creature, and think how many causes we have to think ill of ourselves; and finally we must consider that charity embraces the very lowest human being. It acknowledges that in the sight of God the contempt that we indulge for others has in its very nature a harshness and arrogance opposed to the spirit of Jesus Christ. The true Christian is not insensible to what is contemptible; but he bears with it.
Because others are weak, should we be less careful to give them their due? You who complain so much of what others make you suffer, do you think that you cause others no pain? You who are so annoyed at your neighbor’s defects, are you perfect?
How astonished you would be if those whom you cavil at should make all the comments that they might upon you. But even if the whole world were to bear testimony in your favor, God, who knows all, who has seen all your faults, could confound you with a word; and does it never come into your mind to fear lest He should demand of you why you have not exercised towards your brother a little of that mercy which He, who is your Master, so abundantly bestows on you?"