A believer is a follower of a particular religion or spiritual practice. Whether you are a Hindu, a Pagan, or a Catholic, you can call yourself a believer.
Most believers have specific religious beliefs, but you can be a secular believer as well — for example, you might be a firm believer in socialist economic policies or a believer in the benefits of homeschooling. Basically, anyone who holds a strong belief in the truth of something is a believer. The verb believe is at the heart of believer, and it stems from the Old English belyfan, "to believe."