As Jesus entered into the region of the Gergesenes, He encountered two men possessed of fierce devils that cried out, "What have we to do with Thee, Jesus, the Son of God? art Thou come hither to torment us before the time?" So....The devils bear testimony: first, that they are indeed destined to suffer eternal torment on that Great Day when our Lord shall return with glory to judge the living and the dead. Until that Day, God has allowed Satan and his fallen angels (the demons) to tempt the faithful, that by resisting this temptation we might prove ourselves worthy of the heavenly Kingdom.
But by the providence of God, Satan's days are numbered, and the demons are painfully aware of that. They are desperate, therefore, to destroy as many human souls are they possibly can within the allotted time. Thus, Satan prowls the earth like a ravenous lion, "seeking whom he may devour."
Secondly, the devils confess that Jesus is, indeed, the Son of God, Who shall begin His eternal reign at the end of time. It is significant, then, that "even the devils believe...and tremble," while the atheist foolishly claims to deny God's very existence. As the Psalmist declares, "The fool says in his heart, there is no God." There are, however, very few true atheists. Those who claim to be such have for the most part willfully blinded themselves to His presence, using their alleged atheism as a cloak for their sinful and self-indulgent way of life. For as Dostoyevsky said, if there is no God, all is permitted.
Of course, if we don't believe in God, it makes no sense to believe in Satan either. Yet many folk who claim to believe in God deny the existence of Satan, and thus become easy prey for the insidious suggestions of the demons. Moreover, the deceptive lies of the demons are often so subtle that they are well nigh impossible to detect unless are hearts have been purified by divine grace and we have attained at least a modicum level of discernment. In The Screwtape Letters, the senior devil explains to his pupil that Satan's greatest triumph in modern times has been to convince the vast majority of people that Satan does no exist, or that he is merely a comic figure with a long tail and a pitchfork.
As Christians, we are called to put on the whole armor of God, that we might stand aright, resisting by the grace of God every temptation of the Evil One. We are called to spiritual warfare as soldiers in the militia of Christ. If we truly believe in God and strive to obey His commandments, Satan can have no power over us...unless we voluntarily give him this power. Still, demons have thousands of years of experience and their deceptions are so subtle, we can so very easily be led astray without even knowing it. We must be ever vigilant, therefore, and not let down our guard even for a moment...lest we become like those swine who were driven by the demons to run heedlessly over the cliff unto their own destruction.