The Imitation of Christ, by Thomas a Kempis, tr. by William Benham [1886], at sacred-texts.com
By two wings is man lifted above earthly things, even by simplicity and purity. Simplicity ought to be in the intention, purity in the affection. Simplicity reacheth towards God, purity apprehendeth Him and tasteth Him. No good action will be distasteful to thee if thou be free within from inordinate affection. If thou reachest after and seekest, nothing but the will of God and the benefit of thy neighbour, thou wilt entirely enjoy inward liberty. If thine heart were right, then should every creature be a mirror of life and a book of holy doctrine. There is no creature so small and vile but that it showeth us the goodness of God.
2. If thou wert good and pure within, then wouldst thou look upon all things without hurt and understand them aright. A pure heart seeth the very depths of heaven and hell. Such as each one is inwardly, so judgeth he outwardly. If there is any joy in the world surely the man of pure heart possesseth it, and if there is anywhere tribulation and anguish, the evil conscience knoweth it best. As iron cast into the fire loseth rust and is made altogether glowing, so the man who turneth himself altogether unto God is freed from slothfulness and changed into a new man.
3. When a man beginneth to grow lukewarm, then he feareth a little labour, and willingly accepteth outward consolation; but when he beginneth perfectly to conquer himself and to walk manfully in the way of God, then he counteth as nothing those things which aforetime seemed to be so grievous unto him.