We are commanded to know how to answer heresy

from Daas Torah page 192

Rashbatz (Magen Avos 2:19): … The Sages have commanded that we diligently learn Torah in order that we should be able to refute their claims. In the time of the gemora these apikorsim would antagonize the rabbis with their claims from the Bible. Our Sages commanded that we be diligent in learning Torah in order to be able to refute the claims of the apikorsim. In addition, it was necessary to decree a special beracha against heretics in the Amida - Berchas HaMinim. Furthermore, we have been commanded to know how to answer heretics. Therefore, we have assumed a leniency to learn their wisdom so that we can demonstrate to them on their own terms that they have no refutation of Torah. This study of their wisdom is not related to the warning that “whoever reads the external books has lost his portion in the World to Come.” That prohibition only refers to their books which are lacking in wisdom and are just a waste of time. Similarly, the prohibition against learning Greek wisdom does not refer to intellectual endeavors but refers to a specific skill, learned in those times, of speaking in hints. Even that would be technically permitted except for a specific historical occurrence that led to a decree against it. … Therefore, those books that are based upon logical proofs were not included in the prohibition. Someone who studies them can accept that which is true and should endeavor to refute that which is against the Torah. This is what we find with Rabbi Meir who learned from Elisha ben Abuyah who had become a heretic. The gemora says it was comparable to a pomegranate. He ate the valid content and threw away the peel…”