unfailing

Synonyms: absolute assiduous bottomless boundless ceaseless come-through consistent constant continual continuing continuous counted on delivering dependable diligent dyed-in-the-wool endless eternal faithful inexhaustible infallible invariable loyal never-failing persistent reliable rock solid same solid staunch steadfast straight sure surefire there triedand-true true trustworthy unflagging unlimited unrelenting

Antonyms: failing impermanent uncertain unsure