Creer - prepositions

Question

What prepositions can be used with creer and how do they change the meaning.Creer en/a ...

Answer

Hola Emanuel
Creer can be used with "en" for "to believe in something/someone", for example:

Creo en la religión católica. ( I believe in Catholicism.)
Ella cree en la reencarnación. (She believes in reincarnation.)

Here you are believing in "something", e.g. a particular faith, an abstract idea,...
But you can also use it with people with both "en" and "a". There is a difference though. For example:
Marcos creyó en ella cuando nadie la apoyaba. (Marcos believed her/trusted her/had faith in her when nobody supported her.)
but 
Marcos creyó a Cristina. (He believed Cristina [he believed in Cristina's words, what she said])
We could say that "creer en alguien" is believe in something/someone and "creer a alguien" is believe someone.
I can't think of any other prepositions used with creer.
I hope this helps,
Saludos
Inma