The question is obvious, how could the Jewish people who had just seen all the ניסים and were mekabel the Torah turn to avoda zara so soon after?
The Beis Halevi explains as follows. Beni Yisrael really made the עגל l'shem shamayim. They were looking for a way to worship Hashem after they thought that Moshe Rabbenu was gone. They thought that they could make up their own way of worshipping Hashem and use the עגל as an intermediary/replacement for Moshe Rabbenu. They were not worshipping the עגל per se, rather they wanted to worship Hashem through the עגל. They wanted to invent a new way of worshipping Hashem not found in the Torah. This was exactly their sin. After Matan Torah, we are not allowed to create new ways to worship hashem. Even more then that. The Beis Halevi says, eveen if a person knows that doing x will do great things in shamayim, if it is prohibited by the Torah he cannot do it. This is contrast to pre matan torah where the Avos were allowed to be creative in their worship. One answer for how was Yaakov allowed to marry 2 sisters is that he knew that for him it was the right thing, that marrying 2 sisters is what he needed to do to carry out his mission. Since this was pre matan torah he was allowed to do that.
This Beis Halevi is very important for our times. What we need to do is serve Hashem not feel good or spiritual. Today, people want to feel good/spiritual. There is nothing with wrong with that, but it has to take a back seat to serving Hashem and keeping the Torah. There is absolutely no heter to violate even the smallest issur for any "spiritual" activity.
This is one of RHS's points against many of the innovations in certain MO circles. Instead of focusing on avodas hashem and what the Torah says the person needs to do, a focus is put on the person feeling spiritual (והמבין יבין).