I do remember I've read somewhere that modern Christians are no longer required to believe in Hell. Like, it's optional. I've now googled but no such info appeared again. Therefore asking knowlegeable people here - which Church document says the Hell is optional if it is really so.
If you consider the words of Christ as authoritative for describing Christianity, I'd say "Ya." (See Matt. 5:22, 29, 30, 10:28, 18:9, 23:15, 33, etc.)
But of course, you can call yourself a Christian and believe whatever you want. There don't seem to be any "Christianity police" regulating what people associate with the faith.
Of course, it is also possible to be a Christian and be ignorant of the doctrine of hell, which doesn't necessarily preclude you from being a Christian. (That actually seems to be the norm.)
As one of the comments mentioned your probably thinking of "Love Wins". In the book, he does not deny the existence of hell as a real place , rather what he is questioning the nature of hell.
So in the same way, to answer your question, I do not believe there is a major branch of Christianity where the existence of Hell is optional, only the nature of what hell is, is being debated (i.e. who goes there, nature of the punishment, does it last forever or just a time, are the damned forgiven at some point or are they completely destroyed, etc).
Also check out "Sense and Nonsense about Heaven and Hell" by Ken Boa on this topic, its a short and great read on the subject.
Any student of the Bible should undertake very seriously to understand the passages in Jas' answer. Do those passages represent the truth? Yes. Is passing a test on those passages a condition of salvation? No.
There's a big difference between what is the necessary condition for salvation, and the body of doctrine a teacher should teach. There are several places in the Bible which offer very simplistic formulas for salvation. For example:
If we take this passage to be literally true, it is hope to the person who just grasped some doctrine, but probably cannot recite and defend doctrine at the level of, say, the Athanasian Creed. This passage also does not say, "Teachers, be satisfied that you're done when these two conditions are met."
As I said above, there is a big difference between teaching something false and not understanding all of doctrine.
1) Biblical evidence:
2) Church history evidence:
Martin Luther's 95 Theses expresses all kinds of anathemas to the hierarchy of the Roman Catholic Church at the time. But very few make any negative reference to the rank-and-file Catholic pew sitters at the time. The worst accusation is that they might trust in the Pope's prescribed works and "lose the fear of God through them."