"Keep therefore and do them; for this is your wisdom and your understanding in the sight of the nations ..." (Deuteronomy 4:6)
Chazal (the sages of the Talmud) have taught us that this verse refers to the calculation of the "seasons and constellations", that is to say the determination the beginning of the Jewish lunar month, when the moon begins to grow again after a period of two weeks during which it decreases. This moment is the "filling of the imperfection of the moon so it doesn't show its lack and that its light be like the sun" (from the prayer of Kiddush Levanna - the sanctification of the moon, version sefard).
The cycle of growth and decrease of the moon is an allusion to the concept of Emunah (faith) of each person. In fact, when the moon is full, it can be compared to a complete Emunah, devoid of the slightest lack, of the smallest imperfection. In this case, the light of the moon (the Emunah, faith) become like the light of the sun (the Creator).
A complete Emunah means that we must have a strong conviction in relation to our belief in Hashem. Emuna begins when the human intellect reached the limits of understanding. It is precisely in these situations that we must not allow ourselves to be besieged by doubts, questions apparently without answers.
When a person reaches the level of complete Emunah, without any imperfection, his intellect illuminates him as if he was seeing and understanding, really. This is the essence of complete Emunah: being like a person who sees with his eyes the answers to the difficult questions that arose. This concept refers to the light of the moon which is the equivalent the light of the sun.
Doing the mitswoth ... simply
"The Lord did not set His love upon you, nor choose you, because you were more in number than any people; for you were the fewest of all peoples" (Deuteronomy 7:7)
This verse is understood in light of the commentary of Rashi, according to whom G-d told the people of Israel: "You have showed self-effacement when I gave you some good." This means that when G-d shines on the Jewish people, it does not stray on the paths of intellectual thinking about the Emunah (faith). Rather, the Jewish people feels invigorated by its strengthening in its faith and takes the opportunity to seek Hashem with even more energy.
This corresponds to what is written in the verse (Song of Songs 1:4): "Draw me after You, we will run!" This means we run behind Hashem as soon as He draws us to Him. However, shortly after starting to look for G-d, He seems to elude us and to disappear from our view. This concept is (ibid. 8:14): "My Beloved fled ..." and (ibid. 5:6): "I opened to my Beloved; but my Beloved had turned away, and was gone ... "
At this moment, we must undertake a measured and methodical search of Hashem. We must seek Him and serve Him with actions and good deeds. All this must be done in joy ... until we ultimately will deserve to find Him. Then, we will find Him and acquire a revelation even more wonderful than we had previously attained.
This corresponds to what is written (Exodus 24:7): "All that the Lord has said will we do, and comprehend." That means we will immediately do - without asking questions asked - what G-d commanded us and then we will understand the meaning of each thing. In fact, it is by doing the mitswoth with Emunah, simplicity and joy that we deserve eventually to understand and achieve high and wonderful concepts.
However, apostates and heretics wish to "understand" before "doing" because they are slaves to their intellectual limitations, their desires for material and their bad traits. For them, rising spiritually is unattainable. They immediately want to have the answers to their philosophical questions about the existence of the Divine and the reasons underlying the holy mitswoth. Only in that case they can - possibly - rush to do a mitzvah: if they perceive its spiritual light at its roots and which illuminates all the spiritual worlds.
(Adapted from Likutey Halachot, Rabbi Nathan of Breslov, Orach Chaim, Hilkhot Nefilat Apayim Ou'Qédoucha DeSidra 4:13, according Otsar Ha-Ira, Emunah 17)