faithful (adj.)

early 14c., "sincerely religious, devout, pious," especially in reference to Christian practice; mid-14c., "loyal (to a lord, friend, spouse, etc.); true; honest, trustworthy," from faith + -ful. From late 14c. in reference to a tale, a report, etc., "accurate, reliable, true to the facts." The noun sense of "true believer, one who is full of faith" is from late 14c. (Church Latin used fideles in same sense). Related: Faithfully; faithfulness. Old Faithful geyser named 1870 by explorer Gen. Henry Dana Washburn, surveyor-general of the Montana Territory, in reference to the regularity of its outbursts.

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Definitions of faithful from WordNet
faithful (adj.)
steadfast in affection or allegiance;
years of faithful service
we do not doubt that England has a faithful patriot in the Lord Chancellor
faithful employees
faithful (adj.)
marked by fidelity to an original;
a faithful copy of the portrait
a faithful rendering of the observed facts
Synonyms: close
faithful (adj.)
not having sexual relations with anyone except your husband or wife, or your boyfriend or girlfriend;
he remained faithful to his wife
faithful (n.)
any loyal and steadfast following;
faithful (n.)
a group of people who adhere to a common faith and habitually attend a given church;
Synonyms: congregation / fold