Please, take a look at: https://drive.google.com/open?id=1lI653IW5Fi5WAXudqCZOppaZEf_H1-2N
There's the fixed idiomatic phrase: Je n'y crois pas. -- I don't believe it. But in general, the verb croire can go with a simple direct object (without preposition) but it can also use à or en as prepositions with a slight shift in nuance.
croire qqc -- to believe something. Tu crois qu'il arrive? - Oui, je le crois. Here le is required to refer back to the idea mentioned in the previous question.
I hope that helps.
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