noun

the quality of being faithful; fidelity.

faithfulness in marriage

fidelity,loyalty,constancy,devotion,trueness,true-heartedness,dedication,commitment,allegiance,adherence,dependability,reliability,trustworthiness,staunchness,steadfastness,fealty,accuracy,precision,exactness,closeness,strictness,fairness,justness,factuality,truth,truthfulness,veracity,authenticity,veridicality