Somewhere in the past, the society of Anthem went through a change due to a war between groups. One group was called the rebels, and their fight for independence from the collective society that Equality lives in was squashed. After this Great War, also known as the Unmentionable Times, the dystopian society burned all of the books and knowledge of the past during an incident called The Great Burning. They determined that in order to have a perfect society (utopia), its citizens must think that the group is more important than the individual. In the society, it is a sin to be smarter than everyone else. Everyone is "equal"; no one is better than the other. Equality is above and beyond his fellow citizens in that he is smart and curious. He is unhappy with his life especially when he is given the job of street sweeper by the Elders. He discovers marvelous inventions from the past including books and electricity in an underground subway system. It is then that he finds the word, "I". He begins to write down his experiments and feelings about the society that he lives in noting how unhappy his fellow man is. He takes his knowledge of the past to the Home of the Scholars where he is rejected. Equality then runs away before he can be arrested to start a new life that cherishes individuality.
It is a sin to write because the society has destroyed all knowledge of the past, and they feel that if people expressed their individuality through writing that it would somehow ruin the society that believes that the group is more important than the individual.