Learning from a teacher who is not fit

 Maharal (Nesivos HaTorah 1:14):It is prohibited to learn from a teacher who is not fit. However, this is not a valid objection since the prohibition applies only to learning from a heretic in person. It is only close personal contact that is prohibited and thus reading a book composed by a heretic would not be present this problem. Nevertheless the question remains whether it is permitted to study their books when they contain attacks against the Torah concerning such thing as the Creation of the world, G‑d’s knowledge, survival of the soul after death and whether the World to Come exists. Perhaps they should be prohibited because they might be a harmful influence? ... However Avos (2:14) says that one must know how to respond to heretical views and if one has not been exposed to heresy how would it be possible to respond to these views? Obviously, it is necessary to be aware of the views of heretics. However, this is obviously permitted only if the intent is to learn their views in order to be able to refute them. If he has this motivation, then it is permitted to read their books and there is no need to avoid them out of the concern of being influenced. However to learn their books and quote their views in order to explain Torah when these heretics have no portion in the Torah—the name of the wicked is to be obliterated... Recently these types of heretical works have circulated and they have negatively influenced people even concerning the foundations of faith... However, if the discussion found in these works supports and reinforces the words of our sages then it appropriate to accept. However if it against our sages even in the slightest, G‑d forbid that it be accepted at all. The general rule is to study their words in order to be able to refute their criticisms…. One should always be careful and diligent with his entire being to establish the truth. This is what our Sages commanded forcefully that one should be very diligent in their studies to learn what to reply to the heretic… It is obvious that this is not limited to Greek philosophy in which they analyzed things entirely with their intellect. In fact, it is a caution to be very diligent and to think carefully to be able to reply to their assertions in order to establish the true religion…