http://www.learnersdictionary.com/search/certain certain
2 — used with it to say that something is known to be true or correct
? Although little is known about these ancient people, it is certain that they grew a number of crops.
? It's not certain [=definite] that the company will lay off workers. [=the company may lay off workers, but it also may not]
? It seems certain that the jury will convict him.
Would it be correct to say "It is sure that..." to mean "It is certain that..."?