Believe (in)

I believe you.
I believe in you.

Is there a difference? It makes a world of difference with or without just a single word.
The former refers to a matter of trust. It implies that you are not lying and are speaking the truth.
The latter implies that I have faith in you (and your abilities), that I know you are capable of succeeding in whatever needs to be done.

Sometimes even native speakers of English make errors regarding the use of words like "in".
As they say, English is easy to learn but hard to master. However, it does not mean that we should abandon hope. Rather, we should believe in ourselves, and repay that belief by endeavoring to improve our language ability little by little, day by day.