From Unseen Warfare:

You must never be afraid, if you are troubled by a flood of thoughts, that the enemy is too strong against you, that his attacks are never ending, that the war will last for your lifetime, and that you cannot avoid incessant downfalls of all kinds. Know that our enemies, with all their wiles, are in the hands of our divine Commander, our Lord Jesus Christ, for Whose honour and glory you are waging war. Since He himself leads you into battle, He will certainly not suffer your enemies to use violence against you and overcome you, if you do not yourself cross over to their side with your will. He will Himself fight for you and will deliver your enemies into your hands, when He wills and as He wills, as it is written: ‘The Lord thy God walketh in the midst of thy camp, to deliver thee, and to give up thine enemies before thee’ (Deut. xxii, 14).

+ From Unseen Warfare, St. Theophan the Recluse and St. Nicodemus of the Holy Mountain