J. I. Packer on two ways we can fall into error when we think about the devil:
[W]hen we study demonology we walk on a knife-edge; at our feet all the time are two yawning chasms of error, into which we can all too easily topple. On the one hand, we can take Satan too seriously, as some in the early church and the Middle Ages did. This will cause us to fall out of the peace of God into morbid fears and fancies: the devil will become the main theme of our theology, and we shall take up a negative view of the Christian life as primarily a course of devil-dodging exercises and anti-Satanic manoeuvres. This was the mistake that led men to become monks and hermits in the early church: they withdrew from ordinary life in order to fight the devil full-time and without distraction, believing that they could not otherwise keep clear of his clutches. The root of their mistake was unbelief; they would not trust God to keep them safe if they stayed in the world (see John 17:15). They were vividly aware that the devil is an adversary of terrible malice and great power, but they failed to realize that by virtue of Christ’s cross he is now a defeated foe. The biblical answer to their fears was given by the Reformers and Puritans, who without minimizing in the least the devil’s ferocity against the people of God, offered a worthy exposition of the triumph of Calvary, the scope of Christ’s promises and the reality of his keeping power.
On the other hand, we can also err by not taking the devil seriously enough. This … is the characteristic mistake of modern times. The denial that Satan is a personal agent is an extreme form of it. Unwillingness to take the devil seriously has two bad effects: it fools men, by keeping from them the knowledge of their danger as objects of the devil’s attacks, and it dishonours Christ by robbing the cross of its significance as a conquest of Satan and his hosts (cf. Col 2:15).
From the chapter on The Devil in 18 Words: The Most Important Words You Will Ever Know