Synonyms for constancy
noun
Faithfulness
fidelity, attachment, adherence, fealty, allegiance, devotion, ardor, eagerness, zeal, passion, love, earnestness, steadfastness, permanence, staunchness, principle, integrity, honor, faith, honesty, devotedness, abidingness, endurance, dependability, trustworthiness, trustiness, unchangeableness, consistency, regularity, certainty, unfailingness; see also loyalty, reliability.
Antonyms faithlessness, disloyalty*, perfidy.
Determination
resolution, perseverance, doggedness, firmness; see determination 2.
See constancy in Webster's New World Roget's A-Z Thesaurus II
Learn more about constancy