Matthew 25:46 where it is said that the rejected go away to eternal punishment, and the righteous to eternal life.
The Greek word for punishment is kolasis, which was not originally an ethical word at all. It originally meant the pruning of trees to make them grow better. I think it is true to say that in all Greek secular literature kolasis is never used of anything but remedial punishment. The word for eternal is aionios. It means more than everlasting, for Plato - who may have invented the word - plainly says that a thing may be everlasting and still not be aionios. The simplest way to out it is thataionios cannot be used properly of anyone but God; it is the word uniquely, as Plato saw it, of God. Eternal punishment is then literally that kind of remedial punishment which it befits God to give and which only God can give. http://www.auburn.edu/~allenkc/barclay1.html
