R Moshe here in Kol Rom II says that the passuk means that we should never decide what form our avodas Hashem should take, because human philosophy, undirected, will inevitably err and begin a false trajectory that will ultimately lead to the worst abominations. The originators of Moleich worship may have been fine people who sought to express their awe and love for their Creator, but they ended up killing their own children without seeing that as a contradiction to their desire to become better people. We have Maseches Derech Eretz to teach us manners, and we should never look to the goyim, or even within ourselves, to decide what is good. Even when we seek to do good and be civil and just, incremental error will creep in and ultimately bring us to very bad ends.
חלק ג, דרשות, דרוש יב
According to Reb Moshe, the issur and the connection to Molech is understandable. Humans, over time, inevitably have moments when we fool ourselves and rationalize desires that stem entirely from self interest. We tell ourselves that they are le'sheim Shamayim when they are not. Unless there are clear guidelines of what is appropriate, terrible things will creep in and ruin whatever good originally motivated us. Unlike science, which usually reveals error, religious philosophy, even if initially well motivated, will veer off into the most terrible behavior. But the Beis HaLevi is hard to understand.
How can the Beis HaLevi say that all Gentile religious behaviors are not only wrong but even motivated by an antipathy to the Ribono shel Olam? There are three things I want to say about the Beis Halevi. One is introductory, and two are explanatory.
Before even addressing the question, you have to realize that many things that we see in other religions stem from our Mesora. Christianity and Islam intentionally imitate and incorporate some elements of Judaism, as does Reform "Judasim." To the extent that they are loyal to their original inspiration, what they do might have legitimacy. The Beis HaLevi is talking about religions that have no shaychus to Judaism.
To explain why, in those cases, he says that all that they do is a mask for Rishus, here are two explanations.
1. Because we believe that not recognizing the Ribono shel Olam is the same as knowing and denying. The person must have a reason to refuse to accept what is, to the honest thinker, self evident. If so, all his religious behaviors grow from a poisonous root.
2. Man-made religion is just another way to give voice to the id without creating tension with the ego. It enables a person to do what he wants to do without his conscience bothering him. Behaviors that any normal human being would be ashamed of, become a mitzva when they are re-cast as religiously motivated. By declaring ourselves to be in the pursuit of some great and worthy goal, we free ourselves from the constraints of our consicence. Human-sourced religion serves only man, and it ultimately serves the worst traits in man. Only divinely inspired religion serves God.
A moment of thought will show how true this is. Chazal tell us that our Toras Emes can turn into a sahm hama'ves. If so, kal vachomer ben beno shel kal vachomer other religions.