1. Believe in sth
We use IN with BELIEVE in the following two meanings:
A) Accept that something exists; e.g.
Do you believe in ghosts?
B) Think that something is good; e.g.
a) I don’t believe in capital punishment. (mean Capital punishment is not a good thing in my view.)
b) I believe in getting a good night’s sleep before an examination.
2. Believe sth
We don’t use any preposition with BELIEVE in the following meaning: Accept that something is true or real; e.g.
a) I believe every word she said.
b) I couldn’t believe what I was seeing.
INCORRECT: You should not believe in everything you hear.
CORRECT: You should not believe everything you hear.
INCORRECT: She couldn’t believe in what was happening to her.
CORRECT: She couldn’t believe what was happening to her.
INCORRECT: I could hardly believe in my eyes.
CORRECT: I could hardly believe my eyes.