faithful

1. Adhering firmly and devotedly, as to a person, cause, or idea; loyal.

2. Engaging in sex only with one's spouse or only with one's partner in a sexual relationship.

3. Having or full of faith.

4. Worthy of trust or belief; reliable.

5. Consistent with truth or actuality: a faithful reproduction of the portrait.

1. The practicing members of a religious faith, especially of Christianity or Islam: a pilgrimage to Mecca made by the faithful.

2. The steadfast adherents of a faith or cause: a meeting of the party faithful.

Synonyms: These adjectives mean adhering firmly and devotedly to someone or something that elicits or demands one's fidelity. Faithful and loyal both suggest undeviating attachment, though loyal applies more often to political allegiance: a faithful employee; a loyal citizen.
True implies steadiness, sincerity, and reliability: "I would be true, for there are those who trust me" (Howard Arnold Walter).
Constant stresses uniformity and invariability: "But I am constant as the northern star" (Shakespeare).
Fast suggests loyalty that is not easily deflected: fast friends.
Steadfast strongly implies fixed, unswerving loyalty: a steadfast ally.
Staunch even more strongly suggests unshakable attachment or allegiance: "He lived and died a staunch loyalist" (Harriet Beecher Stowe).