Memetics is a comparison of memes (ideas) to genetics. Ideas will pass from one person to another - religion is a meme.

Hell is a very effective meme, if you can convince people that not believing will send them to hell, then that person has inherited the meme, which lends itself to being spread by the people it is spread to.

It is a very unfair meme, in that it attempts to force belief in the accompanying meme through fear-mongering.

Think about hell logically for a second - any God who would be willing to subject people to infinite punishment for all eternity is cruel and sadistic. It is fear-mongering, not the realistic actions of a supposedly kind and loving god. [More info here]

Religion can be compared unfavorably to a virus - it spreads itself using new hosts to spread to more hosts.

When you consider that those that spread the meme often spread it to their children as well - inspiring obedient faith to a religion via fear - the whole picture becomes clear.

Hell is imaginary, yet, people use it as a reason to spread the religion, as well as a rationalization for the spreading of it and an argument used while spreading it.

Cruel invention indeed.