Thomas á Kempis. The imitation of Christ. Book 2, chapter 4.

Of a pure mind and simple intention

We are raised up from the earth by two wings—simplicity and purity. There must be simplicity in our intention and purity in our desires. Simplicity leads to God, purity embraces and enjoys God.

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 is right, every created thing will 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 are good and pure, you will see all things clearly and understand them rightly, for a pure heart penetrates to heaven and hell; as you are within, so you will judge what is without. If there is joy in the world, the pure of heart certainly possess it; and if is anguish and affliction anywhere, an evil conscience knows it too well.

As iron cast into fire loses its rust and becomes glowing white, so whoever turns completely to God is stripped of sluggishness and changed into a new being.