Enchiridion militis Christiani, or Handbook of a Christian Knight (or: Soldier) was written by Desiderius Erasmus in 1501 and was published in English in 1533 by William Tyndale.
During a stay in Tournehem, a castle near Saint-Omer, Erasmus encountered an uncivilized, yet friendly soldier who was an acquaintance of Battus, Erasmus' close friend. On the request of the soldier's pious wife, who felt slighted by her husband's behaviour, Battus asked Erasmus to write a text which would convince the soldier of the necessity of mending his ways, which he did. The resulting work was eventually re-drafted by Erasmus and expanded into the Enchiridion militis Christiani, The Enchiridion is an appeal on Christians to act in accordance with the Christian faith rather than merely performing the necessary rites. It became one of Erasmus' most influential works.