faith·ful 
(fāthfəl)
(fāthfəl)adj.
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.
a. Responsible; conscientious: the faithful discharge of his duties.
b. Dependable; reliable: The faithful engine started right up.
4. Consistent with truth or actuality: a faithful reproduction of the portrait.
5. Having or full of faith.
pl.n.
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.
faithful·ly adv.
faithful·ness n.
Synonyms: faithful, loyal, true, constant, steadfast, staunch1
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: remained true to her innermost beliefs. Constant stresses uniformity and invariability: "But I am constant as the northern star" (Shakespeare).
Steadfast implies fixed, unswerving loyalty: a steadfast ally. Staunch even more strongly suggests unshakable attachment or allegiance: a staunch supporter of the cause.
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: remained true to her innermost beliefs. Constant stresses uniformity and invariability: "But I am constant as the northern star" (Shakespeare).
Steadfast implies fixed, unswerving loyalty: a steadfast ally. Staunch even more strongly suggests unshakable attachment or allegiance: a staunch supporter of the cause.