Why does it say "Sod Hashem li'Re'av"?
Radak: Hashem reveals His secret only to those who fear Him and are complete in [guarding His] Mitzvos.
Malbim: This that the Divine Nefesh will dwell in good in the reward of the world to come, is Hashem's secret to those who fear Him. They know the secret of the Nefesh and its matter, and that a moment of contentment in the world to come is better than all life in this world.
What is the meaning of "u'Vriso Lehodi'am"?
Radak: He reveals His secret to them. He carves the Bris with the Neshamah, to make it wise and gather it to His honor when it will separate from the body. Also to Moshe, He said "Hinei Anochi Kores Bris Neged Kol Amcha E'eseh Nifla'os 1 " (Shemos 34:10).
Malbim: This corresponds to "v'Zar'o Yirash Aretz" (verse 13). Via this, Hashem informs those who fear Him of His Bris, just like He made a Bris with the Avos that their seed will inherit the land, and with David, that kingship will not cease from his seed.
Radak: The Bris was to grace Moshe with the light of intellect, and his Nefesh clings to upper beings. No one else recognized this. "Neged Kol Amcha", I will make a physical sign that all will see - your face will radiate - from which people will recognize that I [also] graced you with a spiritual sign.
