Be certain of is a phrase that encourages one to have confidence in their decisions and actions. The two crossword solutions for this phrase are 'KNOW' and 'RELY'. Knowing something means to be aware of it, to understand it, and to have knowledge of it. Relying on something means to trust it, to depend on it, and to have faith in it. Together, these two words suggest that one should have confidence in their decisions and actions.