From The Imitation of Christ:
Purity of Mind and Unity of Purpose
A MAN is raised up from the earth by two wings—simplicity and purity. There must be simplicity in his intention and purity in his desires. Simplicity leads to God, purity embraces and enjoys Him.
If your heart is free from ill-ordered affection, no good deed will be difficult for you. If you aim at and seek after nothing but the pleasure of God and the welfare of your neighbor, you will enjoy freedom within.
If your heart were right, then every created thing would be a mirror of life for you and a book of holy teaching, for there is no creature so small and worthless that it does not show forth the goodness of God. If inwardly you were good and pure, you would see all things clearly and understand them rightly, for a pure heart penetrates to heaven and hell, and as a man is within, so he judges what is without. If there be joy in the world, the pure of heart certainly possess it; and if there be anguish and affliction anywhere, an evil conscience knows it too well.
As iron cast into fire loses its rust and becomes glowing white, so he who turns completely to God is stripped of his sluggishness and changed into a new man. When a man begins to grow lax, he fears a little toil and welcomes external comfort, but when he begins perfectly to conquer himself and to walk bravely in the ways of God, then he thinks those things less difficult which he thought so hard before.