Most Catholics deny that Satan, known as Lucifer or the devil, really and personally exists. In denying this Catholic teaching, they weaken or destroy the belief in many other Catholic doctrines such as the eternal punishments of Hell, mortal sin and its consequences, God's final judgment, belief in angels, Original Sin prompted by the devil and the necessity of baptism.
It's a dogma of Catholic faith that Hell was created specifically for the fallen angels or devils. Our Lord Himself tells us this in Matthew 25: 41 where He says, "Then He shall say to them also that shall be on His left hand: Depart from Me, you cursed into everlasting fire, which was prepared for the devil and his angels."
Archbishop Fulton Sheen admonished those who denied the devil exists as a real person. In his book, The Eternal Galilean, Abp. Sheen wrote, "Do not mock the Gospels and say there is no Satan. ... Satan never gains so many cohorts as when, in his shrewdness, he spreads the rumor that he is long since dead."
The problems with denying the existence of the devil are legion. The same study that showed more than eight in 10 Catholics believe the devil is just a symbol also showed these unbelieving Catholics were also less likely to support pro-life initiatives, believe in moral obligations or be religiously active.
If the devil isn't real then Catholic theology falls apart. If the devil never tempted Eve then is there such a thing as Original Sin? If there isn't Original Sin then why did Christ die on the Cross? There'd be no need for a Redeemer to save fallen man from sin nor the necessity of sinful man to be baptized.
As the Church teaches that Hell was created for the fallen angels, then if the devil isn't real then Hell isn't real. If there's no Hell then there are no eternal consequences as a result of dying in a state of unrepentant mortal sin. Without Hell the phrase, "There's a reasonable hope that all men are saved" must be changed to "It's certain that all men are saved." The question would then arise, "Saved from what?"
The first pope of the Church, St. Peter, actually uses the fate of the fallen angels to warn Christians from living a life a sin. In II Peter 2: 4 and 9, he writes:
For if God spared not the angels that sinned, but delivered them, drawn down by infernal ropes to the lower hell, unto torments, to be reserved unto judgment ... The Lord, therefore, knows how to deliver the godly from temptation, but to reserve the unjust unto the day of judgment to be tormented.
Watch the panel discuss how Catholic doctrine relates to Satan in The Download—The Devil's Great Lie.