I read in Hilchos Avodah Zara that for one who claims there is no punishment for transgressing the Torah, it is forbidden to talk to them or answer them. If I am a gentile in the process of Geirus, and I believe my father to fall into this category, would this apply here? He is conservative and not very observant, but he says the Shema and sometimes wears Tefillin, but that is it. If it is forbidden to talk to him and all other of these people, do gentiles who don’t accept the Torah fall into the same category? Is it permitted to do business with the gentiles and not the Jewish idol worshippers/heretics, or both? Thank you.
What the Rambam is referring to is when the non believer is discussing his mistaken ideologies, the Rambam says that we should not discuss these things with them ,because it can cause us to be convinced to their mistakes. The Rambam does however say that we should know the answer to all of their arguments, for our own knowledge, but not to engage them in debates.