Thomas a Kempis, The Imitation of Christ 2.4

 Of a pure mind and simple intention

1. 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 reaches towards God, purity apprehends Him and tastes Him. No good action will be distasteful to you if you are free within from inordinate affection. If you reach after and seek, nothing but the will of God and the benefit of your neighbor, you will entirely enjoy inward liberty. If your heart is 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 shows us the goodness of God.
 

2. If you are good and pure within, then you will look upon all things without hurt and understand them rightly. A pure heart sees the very depths of heaven and hell. Such as each one is inwardly, so judges he outwardly. If there is any joy in the world, surely the man of pure heart possesses it, and if there is anywhere tribulation and anguish, the evil conscience knows it best. As iron cast into the fire loses rust and is made altogether glowing, so the man who turns himself altogether unto God is freed from slothfulness and changed into a new man.

3. When a man begins to grow lukewarm, then he fears a little labor, and willingly accepts outward consolation; but when he begins perfectly to conquer himself and to walk manfully in the way of God, then he counts as nothing those things that previously seemed to be so grievous to him.