Infinite punishment for finite offenses is unjust on the face of it. The biblical support for it is dodgy though, you can put together verses to support the 'Lake of Fire' being reserved for Satan and his fellow rebel angels, annihilation of the souls of sinners, or verses to support eventual universal salvation. Eternal hellfire seems to just be popular in many Christian sects, presumably because fear of it is believed to help keep the flock in line. In modern times the eternal hellfire doctrine is a problem, possibly because the morality of modern culture has moved beyond considering torture a just punishment even if it's temporary.
I'm not anti-Christian. I'm anti-stupid.