The Beis HaLevi explains his idea further: "The idea of Temimus is that a person should perform the will of G-d without questioning why He commanded him so. Although a person is permitted to search for reasons for the Mitzvos, that search itself is part of the Mitzvah of learning Torah, and even that study should be done solely because it is a Mitzvah and the will of G-d, not because one desires to question why Hashem commanded him such and such. A person's essential motivation in performing the Mitzvos, and in learning as well, should be solely because that is the will of G-d."
According to the Bais HaLevi, even those Mitzvos that we understand and appreciate should be performed from a posture of Temimus. Our motivation should be completely and simply "because Hashem instructed us so."
The Midrash Bereishis Rabah (47) comments that the Avos are called the Mercavah (chariot) of Hashem. Sifsei Chaim writes that one aspect of this idea is that when a chariot goes right or left, it is not the decision of the chariot. The driver controls the chariot and his will drives it in whichever direction it goes. So too the Avos HaKedoshim had no will of their own; they were simply carrying out the will of G-d. Their motivation was simply, "because Hashem instructed us so."
To perform Mitzvos that we understand and enjoy, simply because they are the will of G-d, is a lofty level to achieve. It demands that we treat even Mishpatim (laws that we would have introduced ourselves, like the prohibition of killing) as if they were Chukim (laws that we never would have instituted on our own, like the red heifer). It means being sure that any enjoyment involved in a Mitzvah is a result of the Mitzvah performance, and not its motivating factor. While we may enjoy performing certain Mitzvos, that enjoyment cannot be the reason why we did it. However, performing that which we do not understand simply because it is the will of G-d, is a level that we should be able to attain.
In discussing how to react when a person fails to understand something in Torah, the Rambam (Avodah: Hilchos M'eelah 8:8) writes that it is appropriate for a person to delve into the laws of the Torah and understand them in depth to the best of his ability. However, when encountering something that he cannot understand, he should not treat it lightly, nor should he make too much out of it, nor should he treat it like any other mundane, secular topic.
Rav Aharon Kahn SHLIT"A explained that the Rambam is presenting us with three possible reactions a person might have when he doesn't understand a Mitzvah. One response is to say that it is meaningless. If one doesn't understand the Mitzvah to keep Kosher, then one inappropriate response is to say that it is Chas VeShalom meaningless and unimportant in Hashem's grand scheme of things. Another improper response is to say that not only is it meaningful, but it contains deep secrets. Sometimes when people fail to understand something, they make it into the deepest Kabbalistic secret, even when it is not. The final incorrect response is to relate to the Mitzvah in a mundane, secular fashion. There have always been those who took an extreme, rational approach to Mitzvos, and keeping Kosher, for example, became a value solely because it was healthy.
The Rambam continues: Come and see how strict the Torah is with M'eelah. If sticks and stones and dust and ash, once G-d's name is attached to them with words alone, become holy, and whomever relates to them in a mundane fashion transgresses the prohibition of M'eelah, how much more so a Mitzvah that Hashem gave us: a person shouldn't treat it lightly because he doesn't know its reason, and he shouldn't attribute things that are untrue onto Hashem, and he shouldn't relate to them as any other mundane, secular idea.
The Rambam is drawing a Kal VeChomer from the prohibition of M'eelah: Once sticks and stones (objects that possess no inherent Kedushah) become holy, it is prohibited to relate to them in a mundane fashion, how much more so the Torah, which has inherent Kedushah, must not be related to in a mundane way.
While this is a valuable lesson, it seems strange that the Rambam chose to illustrate this idea in Hilchos M'eelah. Why did the Rambam raise the issue of failure to understand Torah on the backdrop of M'eelah? To put the question most sharply: What do these incorrect responses have to do with the prohibition of M'eelah?
I think that the Rambam was being extremely exact in his formulation of this Kal VeChomer and his placement of it in Hilchos M'eelah. The precise prohibition of M'eelah is to receive forbidden personal enjoyment from something that is sanctified to Hashem (Hekdesh). This is true even of sticks and stones which have no inherent sanctity. Once they are dedicated to the Temple, they achieve a status of belonging to Hashem and certain personal enjoyments from them are prohibited. How much more so when it comes to the Torah, which has inherent sanctity (as it is the will of G-d) certain personal enjoyments from it are prohibited. When a person doesn't understand something, what drives him to respond in one of these three ways? Why would he either reject a Mitzvah as meaningless or make too big of a deal over it or simply treat it in a simple rational way?
When a person fails to understand something, he feels incapable. His ego is hurt. After all, how can HE not understand something? In order to restore his ego, a person may downplay the misunderstanding. He might say that the topic is not that important anyway. Alternatively, a person might say, "If I don't understand it, then it must be something really big! It must be a deep Kabbalistic secret!" Or, a person may deal with his incapability by saying, "There is a perfectly rational answer, the Mitzvah is not as complicated as it seems." What all these responses have in common is that they are all ways of repairing a broken ego. They do not reflect an attempt to find the truth of Torah. Instead of saying, "I don't know," and remaining with a feeling of inadequacy, this person skews the Torah to bolster his ego. In other words, he receives forbidden personal enjoyment from that which is inherently sanctified to Hashem - M'eelah.
If we are currently not on the level to do even that which we do understand and appreciate, simply because it is the will of the Almighty, at least we should be able to do that which we don't understand simply because Hashem requested it of us. When we fail to understand something, we should have the humility to admit it and not skew the Holy Torah. We should all merit to perceive the Torah as the will of G-d and perform the Mitzvos when we don't fully grasp them, simply because they are what Hashem wants of us. That is Halichah of Temimus.