Before answering our questions, we turn to a different possuk that shares the usage of the words, this matter: Giving, you shall give him, and let your heart not feel bad when you give him, for because of this matter, Hashem your God, will bless you in all your deeds and in your every undertaking. Rashi, using the alternative rendering for davar – word (rather than matter) – says: Even if you just say that you will give, you are rewarded for the saying as well as the doing.
The Pardes Yosef provides a beautiful explanation: R’ Ovadia Bartenura explains the statement in Avos, "The reward for a mitzvah is a mitzvah” – Whatever joy and fulfillment a person receives from doing a mitzvah is a mitzvah in its own right, so that one is rewarded both for performing the mitzvah and for his joy and pleasure in the mitzvah. The Pardes Yosef explains the above-cited possuk with this idea: Giving, you shall give him, and let your heart not feel bad –but rather feel joy – when you give him. For because of this joy – because of this matter/word – Hashem, your God, will bless you. The same applies to sin, says the Pardes Yosef. Thus, the Midrash explains the double language of the possuk, Yerushalyim sinned greatly (חטא חטאה ): Because they sinned doubly, they were doubly punished. This means, says the Pardes Yosef, that because they took joy in their sinning it counted as a double sin; and when they will repent with joy, that too is counted doubly, and this is the double consolation referred to by the Midrash.
Yet In This Matter You Do Not Believe
We can now apply this idea to our possuk, Yet in this matter you do not believe. Of course Bnei Yisrael were believers, sons of believers, especially the dor deah who had personally witnessed so many miracles in the wilderness. Likewise, Mosheh, after being told by Hashem Himself that they were Believers, sons of believers, could never had said that they were not believers. Rather, he was alluding to the other possuk, Because of this matter... which alludes to performing mitzvos with joy, with a good word.
Thus, in this matter/word you did not believe in Hashem, your God; This matter/word refers to the good word, the rejoicing, to making others happy, in addition to just doing. Mosheh was scolding Bnei Yisrael that on a deeper level they don’t believe in the sense of performing mitzvos with joy and a good heart.
You Cried for Nothing
In a similar deeper level, we can say that on that fateful night when the spies returned, bnei Yisrael truly wanted to follow Mosheh into the Land of Israel. After all, they were believers, sons of believers, and of course they believed Hashem and His servant, Mosheh when they said that it was a land flowing with milk and honey. But they were afraid, and so they cried on that night. Nevertheless, there was no reason to cry, and that was punished with crying for generations. Their punishment was not for not wanting to enter the holy land, but for acquiescing to go with sorrow and tears. Thus, forty years of wandering in the wilderness, the destruction of two Batei Mikdash, and the centuries of exile were all because they did not serve Hashem with joy and a good heart, even though they lacked nothing.
