Guidelines for "becoming" a believer, according to the Catholic Church This might sound like an odd question, but I hope it's on-topic.

Say an atheist/agnostic approaches you and asks, "how can I get to believe in God?". Out of instinct/experience, you can say things like "pray, read, talk to a priest, go to a church, etc". 

I wonder if there is some sort of "official guide" in the Catholic Church directed to non believers, so they can get to know God. I'm thinking in terms of  encyclicals, exhortations, pastoral documents, or even something based on doctrine. Any ideas?
St. Thomas Aquinas's Summa contra Gentiles ("Summary treatise against [the errors of] the pagans") was written precisely to convert those who do not accept the authority of the Christian Holy Scriptures. From bk. 1 ch. 2 ("The Author's Intention in the Present Work") [3]:


  …the Mohammedans and the pagans, do not agree with us in accepting the authority of any Scripture, by which they may be convinced of their error. Thus, against the Jews we are able to argue by means of the Old Testament, while against heretics we are able to argue by means of the New Testament. But the Muslims and the pagans accept neither the one nor the other. We must, therefore, have recourse to the natural reason, to which all men are forced to give their assent. However, it is true, in divine matters the natural reason has its failings.


cf. this bibliography for the English translations of the Summa contra Gentiles