St. Diadochos of Photiki wrote:
Those who are engaged in spiritual warfare must always keep their hearts tranquil. Only then can the mind sift the impulses it receives and store in the treasure house of the memory those that are good and come from God, while rejecting altogether those that are perverse and devilish.
When the sea is calm, the fisherman’s eyes can see the movements of the fish deep down, so that hardly any of them can escape. But when the sea is ruffled by the wind, the turmoil of the waves hides from sight the creatures that would easily have been seen if the sea wore the smile of calm. The skill of the fisherman is of little use in rough weather.
Something of the same sort happens with the soul, especially when it is stirred to the depths by anger.
At the beginning of a storm, oil is poured on the waters to calm them, and in fact the oil defeats their commotion. In this way, when the soul receives the anointing of the gift of the Holy Spirit, it gladly gives in to this inexpressible and untroubled sweetness. And even if it is continually attacked by temptation it maintains its peace and joy.
From Spiritual Works, 23 (SC5b, pp. 27ff.) 5th century