absolutely trustworthy or sure

an infallible rule.

unfailing in effectiveness or operation; certain

an infallible remedy.

not fallible; exempt from liability to error, as persons, their judgment, or pronouncements

an infallible principle.

Roman Catholic Church . immune from fallacy or liability to error in expounding matters of faith or morals by virtue of the promise made by Christ to the Church.

an infallible person or thing.

adjective

erring , fallible , faulty , imperfect

adjective

acceptable , accurate , agreeable , apodictic , authoritative , certain , correct , effective , effectual , efficacious , efficient , exact , faultless , flawless , foolproof , handy , helpful , impeccable , incontrovertible , inerrable , inerrant , omniscient , perfect , positive , reliable , satisfactory , satisfying , sure , surefire , true , trustworthy , unbeatable , undeceivable , unfailing , unimpeachable , unquestionable , useful , secure , unerring , dependable , divine , undeniable