remaining firm, steady, and constant in one's allegiance or support for someone or something, such as carefully fulfilling one's duties/responsibilities or promises;
e.g. The priest has always been faithful to the church and to his god.
(adjective)
reliable or accurate, staying true to a standard or original or to specified facts;
e.g. This is the most faithful replica of the painting that I have seen.
(noun)
a person or group of people closely believing in and following the practices and principles of a particular religion, or a person who is a loyal follower/supporter of a particular political party;
e.g. They believe that every faithful can seek divine blessing.