Croire can be followed by a direct object or an indirect object.
When followed by an indirect object, this object can be introduced by either the preposition à or the preposition en according to what meaning croire has.
English does not make the difference and uses "in" in both cases.
Croire à means to be persuaded in the veracity, the efficiency or the existence of something. it is never followed by a person's name or a pronoun representing a person.
Je ne crois pas aux fantômes. (aux = à les)
I don't believe in ghosts.
Je crois à la parole de Dieu.
I believe in the word of God.
Croire en means to have faith in someone or something and to behave accordingly. It is a spiritual attitude, it can be a sign of love.
Je crois en Dieu.
I believe in God.
Je crois en elle et je sais qu'elle va réussir.
I believe in her and I know she will succed.
When croire is followed by a direct object, that is to say when the complement is not introduced by a preposition, it has the same meaning as the English "believe someone", used also without a preposition.
Je le crois.
I believe him.
Je lui crois. is impossible. You can only use lui with croire en.