While Reiki is spiritual in nature, it is not a religion. It has no dogma, and there is nothing you must believe in order to learn and use Reiki. In fact, Reiki is not dependent on belief at all and will work whether you believe in it or not.
While Reiki is not a religion, it is still important to live and act in a way that promotes harmony with others. Mikao Usui, the founder of the Reiki system of natural healing, recommended that one practice certain simple ethical ideals to promote peace and harmony, which are nearly universal across all cultures.
It is important to keep your spirit healthy in order for Reiki to work the most effectively.