From St. Athanasius’s Life of Anthony:
It is possible, then [the demons] model themselves after the form of monks, for them to pretend to speak like the devout, so that by means of the similarity of form they deceive, and then drag those whom they have beguiled wherever they wish. Nevertheless it is unnecessary to heed them, even if they awaken you for prayer, or counsel you to eat nothing tat all, or pretend to level accusations and reproaches concerning actions for which, at another time, they excused us. They do not do these things for the sake of piety or truth, but so that they might bring the simple to despair, and declare [spiritual discipline] useless, and make men sick of the solitary life as something burdensome and very oppressive, and trip up those who, opposing them, lead it. (Gregg ed., p.50)
It is interesting here that in this description from St. Anthony, the demons do not appear in a form tempting one to sin outright, but rather to place a heavy burden on the victim to do good works, that they might be driven to despair at their failure. The guise here is demon as Puritan – urging you to always be praying and fasting and doing good, beyond what your own psyche and body can handle. Throwing your hands up in the air, you (quite rightly in one sense) declare the whole thing to be an impossible task and give up. But the Lord’s yoke is easy and His burden is light. (Matthew 11:30) Therefore, when we preach a heavy burden of spiritual discipline to our flock or to our children, we must be sure we are not doing the work of demons who would break their spirits and make the Way of Christ seem discouraging and cruel to them. In our present age, this is likely to be a more potent weapon of theirs than nightmares.