Synonyms for never-failing:

adj. reliable

adj. unalterable, definite

adj. intent, resolute; established

adj. infallible

adj. loyal, steady

adj. stable, fixed

adj. certain, definite

adj. sure

adj. certain, unchanging

adj. steadfast

adj. outright, absolute

adj. consistent, unchanging

adj. sincere

adj. enthusiastic, sincere

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