Let not your heart be troubled: ye believe in God, believe also in me.
As concerning therefore the eating of those things that are offered in sacrifice unto idols, we know that an idol is nothing in the world, and that there is none other God but one. For though there be that are called gods, whether in heaven or in earth, (as there be gods many, and lords many,) But to us there is but one God, the Father, of whom are all things, and we in him; and one Lord Jesus Christ, by whom are all things, and we by him.
But without faith it is impossible to please him: for he that cometh to God must believe that he is, and that he is a rewarder of them that diligently seek him.
Often we say "I believe that ..." when we assume sonething, but are not certain about it. When the Bible talks about "believing" something else is referred to: to be certain of something and to hold on to. This does not mean to adhere to an idea or a teaching / doctrine / dogma, but to hold on to a person: God and his promises.