'Believe' and 'believe in'

Is there any differences between 'I believe you' and 'I believe in you'?

Yes, there is, and the difference may be bigger than you thought!

'I believe you' means 'I think you are honest and not lying'.

'I believe in you' means 'I think you are doing a good job and I
have faith in you'.

'Believe' means 'to think that something is true, correct or real'.

'Believe in' is a phrasal verb.

If you believe in something, it means you are certain that something exist, or you are confident that something is effective and right.

If you believe in someone, it means that you trust that person because you think they can do something well, or that they are a good person.

So, here's a challenge! Which of the following is / are grammatically correct and make(s) sense to you?

  1. I believe aliens.
  2. I believe aliens exist.
  3. I believe in aliens.