There are those within Christendom who advocate total annihilation of the sinner who does not repent, and to be honest I believe for the most part they just flat out deny scripture as a whole. Just from these few text of scripture it is crystal clear to me (and most others) that the apostle John believed that the Devil, all false teachers, all unbelievers, death and hell will all be cast into the lake of fire and brimstone and that their torment will be day and night forever and ever, which is clearly the second death!
Matthew 25:41 "Then shall he say also unto them on the left hand, Depart from me, ye cursed, into everlasting fire, prepared for the devil and his angels:"
Revelation 20:10 "And the devil who deceived them was thrown into the lake of fire and brimstone, where the beast and the false prophet are also; and they will be tormented day and night forever and ever."
Revelation 20:14 "And death and hell were cast into the lake of fire. This is the second death."
Revelation 21:8 "But the fearful, and unbelieving, and the abominable, and murderers, and whoremongers, and sorcerers, and idolaters, and all liars, shall have their part in the lake which burneth with fire and brimstone: which is the second death."
I also believe the doctrine of eternal punishment is an essential doctrine of the faith and here is why.
"Those who do not believe in the eternal punishment of the wicked do not believe in the utter sinfulness of sin. They do not believe that sin is that much of an offense to God. And the reason they do not believe that sin is such an offense to the eternal God that the wicked must be punished eternally is because they do not believe in the Holy and Just God of the Bible." —Marc D. Carpenter