incapable of failure, involving no risk

infallible

firmly fixed; loyal

loyal, steady

firmly fixed or supported; not likely to fall over; not changing in amount, speed etc; (of a person behaviour) having, keeping to, a regular (good) pattern

stable, fixed

having good reasons to believe something; free from doubt; proved; reliable; without a doubt

certain, definite

never coming to an end

certain, unchanging

not hesitating

steadfast

with complete commitment, sympathy etc

enthusiastic, sincere