Imitation of Christ

Pone te primus in pace, et tunc alios poteris pacificare. Homo pacificus plus prodest, quam bene doctus. Homo passionatus etiam bonus in malum trahit, et faciliter malum credit. Bonus homo pacificus omnia ad bonum convertit. Qui bene in pace est, de nullo suspicatur. Qui autem male contentus est, et commotus, variis suspicionibus agitatur, nec ipse quiescit, nec alios quiescere permittit. Dicit sæpe quod sibi magis facere expediret, et negligit, quod ipse facere tenetur. Habe ergo primum zelum super te ipsum, tunc zelare poteris etiam juste proximum tuum. 

First keep yourself in peace, and then you will be able to be a peacemaker towards others. A peaceable man does more good than a well-learned. A passionate man turns even good into evil and easily believes evil; a good, peaceable man converts all things into good. He who dwells in peace is  suspicious of none, but he who is discontented and restless is tossed with many suspicions, and is neither quiet himself nor suffers others to be quiet. He often says what he ought not to say, and omits what would be better for him to say. He considers to what duties others are bound, and neglects those to which he is bound himself. Therefore be zealous first over yourself, and then you may righteously be zealous concerning your neighbour.

Imitation of Christ, II, 3:1