The root of this word is aman, a word often translated as "believe" but more literally means "support" as we see in Isaiah 22:23 where it says "I will drive him like a peg in a place of support..." A belief in God is not a mental exercise of knowing that God exists but rather our responsibility to show him our support. The word "emet" has the similar meaning of firmness, something that is firmly set in place. Psalmes 119:142 says, "the 'Torah' (the teachings of God) is 'emet' (set firmly in place).