The main duty of the spiritual man is to keep his mind clean

The main duty of spiritual people is to keep their mind free of intrusive thoughts. This is necessary because the devil is forever seeking a meeting of minds with his prey. Giving in to just a single thought is enough for him to gain a foothold and engage in spiritual combat.
And any mistake which we make in practice has already existed beforehand in the realm of our thoughts.

Based on St. Maximus the Confessor, Abba Mark