To believe sth =
1. To accept as true or real: Do you believe the news stories?
2. To credit with veracity: I believe you.
3. To expect or suppose; think: I believe they will arrive shortly.

To believe in sth= (συνήθως) To have firm faith, especially religious faith.
2. To have faith, confidence, or trust: I believe in your ability to solve the problem.
3. To have confidence in the truth or value of something:
4. to be convinced of the truth or existence of: