Imitation of Christ

Hæc est altissima et utilissima lectio, sui ipius vera cognitio, et despectio. De se ipso nihil tenere, et de aliis semper bene et alte sentire magna sapientia est, et perfectio. Si videres aliquem aperte peccare, vel aliqua gravia perpetrare, non deberes te meliorem exstimare. Omnes fragiles sumus, sed tu niminem fragiliorem te ipso tenebis.

That is the highest and most profitable lesson, when a man truly knows and judges lowly of himself. To account nothing of one's self, and to think always kindly and highly of others, this is great and perfect wisdom. Even if you should see your neighbor sin openly or grievously, yet you ought not to reckon yourself better than he, for you know not how long you shall keep your integrity. All of us are weak and frail; hold no man more frail than yourself.

Imitation of Christ  I, 2:4