Answer

Hello Rabbi A.,
The question is well known and old (similar to the prohibition against returning to Egypt that appears in Maimonides, though he himself lived there), and there is a lot to say about it. I’ll try to present a few points briefly here.
1. From the plain sense of his wording, one can infer that the prohibition is against seriously weighing the alternative, not necessarily against merely studying it in order to broaden one’s mind and deepen one’s faith. By the way, the Tashbetz in Magen Avot writes that this is permitted in order to answer a heretic (apparently including the one inside you. See below).
2. In my opinion, beyond the contradiction within Maimonides, on the logical plane such a prohibition cannot be accepted. It simply cannot be that a system demands that I accept it while at the same time forbidding me from examining the system itself. On what basis am I supposed to accept it if I have not examined alternatives? According to this, any gentile born into an idolatrous nation could be forbidden from examining his beliefs and would remain, against his will, an idol worshiper. Would we then have no complaint against him for not having examined them? Therefore it is clear that such a prohibition cannot make logical sense, and even if such a prohibition exists, I am not willing to accept it. Certainly about this one may say: “By God, if Joshua son of Nun had said it, I would not obey him” (Hullin).
3. It follows that such a prohibition can only be imposed if we are speaking about a person who has already formed a religious position, not someone who is in the process of forming it. After he has decided that he accepts the system upon himself, it may perhaps impose such a prohibition on him (and even that seems very problematic to me. For example, if he again has doubts and wants to reexamine things, there is no logic in forbidding that, for reasons very similar to those above).
4. What reason could there be for a person who already has a position suddenly to want to read about alternatives? Either an evil inclination toward heresy — and that can be forbidden. Or a desire to reexamine things — and in my opinion there is no prohibition against that (as above). Or he wants to become intellectually acquainted with other views and cultures — and in my opinion that too is not prohibited (I once heard that Rabbi Ariel was asked about studying the New Testament as part of history courses and permitted it. You can’t understand Christianity and history without it). By the way, in Maimonides’ own case this is apparently why he did it (at least regarding matters of idolatry — the Sabians, whom he studied. Matters of philosophy are probably something else).
5. According to the “principle of charity” in interpretation, if this is the only logical interpretation of such a prohibition, then it should be read into Maimonides’ words as well (otherwise he is saying something illogical. And as is known, it is preferable to force the language than the reasoning). Especially since his own behavior contradicted his teaching, which strengthens this interpretation even more.
6. I once thought that Maimonides writes this Jewish law because of the following consideration (and I really do not dismiss this, in light of the discussions about esotericism in his writing, especially in the Guide): I will prohibit it halakhically. Anyone who understands that there cannot really be such a prohibition will not obey it anyway (“By God, if Joshua had said it…”). And anyone naive enough (foolish enough) to accept such an illogical prohibition — then really it is good that he refrain from such pursuits (because naive people can fall into the trap, and they do not have sufficient understanding to form a position). For such a person, it is better to rely on the sages and not make decisions on his own.
This is somewhat like writing laws of “but one does not instruct this in practice” or laws meant for zealots (one who cohabits with an Aramean woman) in the Mishneh Torah. If one does not instruct this in practice, then how can he write them there explicitly? (And similarly regarding their appearance in the Talmud.) We are forced to conclude that the Mishneh Torah writes these laws only for those who understand when and whether they are to be applied. Others who read the law innocently and see that one does not instruct this in practice will not instruct it and will not do it. One who understands that there is an absurdity here will also understand how, whether, and when this complex law is to be applied. And so everything works out.
Another example is decrees such as reading by candlelight. Rabbi Ishmael said, “I will read and not tilt [the lamp].” At first glance this is puzzling, because there is a prohibition, and once it was decreed there should be no dependence on the practical concern (for is someone who will never come to eat meat and milk not still forbidden to eat poultry with milk?). This is asked in the book Hazut Kashot at the end of Beitzah. Moreover, even after he retracted, he did not retract his principled position that for someone to whom the concern of the decree does not apply, the decree is not forbidden; he only revised his self-assessment, since he saw that he had almost come to tilt it (and there is a version that says he actually tilted it). Meaning, even in the conclusion he maintained the view that if there is someone for whom there is no concern that he will tilt, the prohibition was not stated regarding him. We are forced to conclude that Rabbi Ishmael held, even according to his final view, that the prohibition is not addressed to one who will not reach the problem that the prohibition came to prevent. And regarding this, several writers on legal principles explained (see Hazut Kashot there, and with Shlomo Tzippinowski, HaTakanot BeYisrael, beginning of volume 1) that in decrees where the reason appears explicitly, one may interpret and limit them in light of the reason (purposive interpretation). And perhaps something similar can also be said about Torah laws, for we do not usually expound the reason of the verse unless the reason is written explicitly in the Torah.
Although this can be discussed in light of the passage in Sanhedrin 20, where it seems that even in such a case we do not expound. But Tosafot HaRosh on Bava Metzia 90 wrote that when the reason is clear, then yes, we do expound; and in a responsum the Rosh wrote this regarding reasons for decrees. And when the reason is written explicitly, it is certainly clear. Now in “do not stray,” the reason is explicit: that you not come to go astray after them. If so, when you need and want to study these matters for serious reasons (not because of impulse), there is no prohibition.
Here I come to the matter of “going astray after them.” For at first glance, even if I study them seriously, it is possible that I will go astray after them. But “going astray” means frivolously, not through serious formation of a position, as will now be explained.
7. I assume you approached me following the short article that was sent out last week from the midrasha/kollel, where I explained that the spies “scouted” the land, and the intention there is light, superficial touring. In contrast, the section about tzitzit comes to teach us to look deeply (the blue thread resembles the sea, which resembles the sky, which resembles the Throne of Glory), “and you shall see them,” and then you will not stumble in “do not stray.” It may be that the entire prohibition against engaging in these areas, which is learned from the verse “do not stray,” speaks of light and superficial browsing. Deep, analytical study of different fields of knowledge is not prohibited at all. The assumption is that you will reach the correct conclusions. But even if not — that is your considered view, and therefore you are under compulsion.