An unbeliever is a person who does not believe in God, or who does not hold a particular religion's faith. Like 'unbelief', this word lives inside religion. And here is the part that matters most for using it safely: it is almost always spoken from inside a faith, looking out. A religious community calls other people unbelievers; people rarely put that name on themselves. Because of that, the word can carry judgement — sometimes gentle, sometimes not. In older writing and in translations of religious texts it can sound quite hard, marking a line between us and them. So if you are describing someone who has no religion, the safe, neutral words are 'atheist', 'non-religious', or 'not a believer'. Save 'unbeliever' for when you are reporting how a religious group talks, or writing about religion. You will also meet it in a wider, joking way in ordinary English, which is worth knowing so it does not confuse you: someone who is passionate about a football team, a band, or a phone brand may call the people who do not share their enthusiasm unbelievers. That use is playful and always borrowing the religious flavour on purpose. It is a countable noun: an unbeliever, two unbelievers.