Zeal is the dog that guards the law of God, which is virtue.
The zealot, as a burning oven, is heated, like the Cherubim, and watches every moment the evils and the attacks of the wicked spirits.
The zeal fades when one is unfaithful to Divine Providence and forgets God.
The coldness of the zeal of virtue comes when the desire for virtue diminishes, and when we trust ourselves and are not afraid of demons.
Saint Isaac the Syrian