The Imitation of Christ, by Thomas a Kempis, tr. by William Benham , at sacred-texts.com
Look well unto thyself, and beware that thou judge not the doings of others. In judging others a man laboureth in vain; he often erreth, and easily falleth into sin; but in judging and examining himself he always laboureth to good purpose. According as a matter toucheth our fancy, so oftentimes do we judge of it; for easily do we fail of true judgment because of our own personal feeling. If God were always the sole object of our desire, we should the less easily be troubled by the erring judgment of our fancy.
2. But often some secret thought lurking within us, or even some outward circumstance, turneth us aside. Many are secretly seeking their own ends in what they do, yet know it not. They seem to live in good peace of mind so long as things go well with them, and according to their desires, but if their desires be frustrated and broken, immediately they are shaken and displeased. Diversity of feelings and opinions very often brings about dissensions between friends, between countrymen, between religious and godly men.
3. Established custom is not easily relinquished, and no man is very easily led to see with the eyes of another. If thou rest more upon thy own reason or experience than upon the power of Jesus Christ, thy light shall come slowly and hardly; for God willeth us to be perfectly subject unto Himself, and all our reason to be exalted by abundant love towards Him.