Oh tepidity! If those who give way to it only rightly understood what it is, they would less easily fall victims to it, for they would dread to become the slaves of so cruel a tyrant. If we are free from this vice, nothing we can do or suffer for God, even death itself, seems too great a burden, whereas the victim of tepidity finds a straw too heavy for him to carry. This vice ruins a man's spiritual life, and it not only stops all perseverance in the good he had commenced, but even causes him to repent of having begun it, thus turning into bitterness what should be sweeter than honey to his soul.
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. Compare the trouble that is undergone by one who serves God diligently and fervently, with that which the tepid soul suffers through sloth, and you will find that the burden borne by tepidity is a thousand times the heavier. It is indeed wonderful that vigils, prayer, fasting, mortification and other works undertaken for our Lord should bring more pleasure to fervent souls than the tepid find in all their feasts, and riches, and other indulgences. The lukewarm Christian appears gay, but grief gnaws at his heart: while the just man, though his life be one of penance, has happiness within his soul.
St. John of Avila, from Letters of Blessed Juan de Avila, pp. 91-92.