Similar word to certainty:

all sewn up, authoritativeness, belief, certitude, cinch, confidence, conviction, credence, definiteness, dogmatism, faith, firmness, indubitableness, inevitability, lock, lockup, open and shut case, positiveness, positivism, rain or shine, setup, shoo-in, staunchness, steadiness, stock, store, sure bet, sure thing, surefire, sureness, surety, trust, validity, wrap-up