From one of his letters:
The Israelites who journeyed through the desert had appetites so disordered that they could not enjoy the manna "containing in itself all sweetness," which God sent them. Their blindness was so great that they did not find fault with themselves, or with the evil condition of their health, but with the food, which was of the most savoury kind. They asked for some other sort of viand with which they thought they would be better satisfied and pleased:—it was given them, but at the cost of their lives. We are to learn by this that even if the things of God are not always agreeable to us, still we must not wish for what is contrary to them, however delightful it may seem to us, for without doubt it would poison our souls. We should rather rid ourselves of the disgust we feel for religion, and then, when the appetites of our soul are healthy, we shall feel a right and pleasant relish for the food God gives His children.
To work slothfully and tepidly in God's service will cause you to lead so unhappy a life that you will be forced to change your ways. Besides, such a life is disloyal to our Saviour Who laboured with such ardent love to redeem us, and so willingly took up the cross that His love for us exceeded His suffering. The tepid soul cannot enjoy the world's pleasures, having given them up in the desire of doing right, and yet, for want of fervour, it does not find happiness in God. In this way such a soul is placed between two opposites, each of which is a torment to it; it suffers such severe afflictions that at last it leaves the right road, and with miserable fatuity seeks the flesh-pots of the Egypt it had left, because it cannot endure the hardships of the desert.