And you shall write on them all the words of this law, when you have passed over; that you may enter the land which Hashem your G-d gives you, a land flowing with milk and honey, as Hashem, the G-d of your fathers, has promised you.
The Rishonim have multiple explanations as to how this Mitzvah will allow them to enter the land. The Ibn Ezra explains that this Mitzvah will be the first of many, and with the many Mitzvos that we keep, we will flourish in Eretz Yisroel. The Ramban writes that the true reason for Klal Yisroel’s living in Eretz Yisroel is to keep the Mitzvos. The Torah that will be written on the Matzevah at the entrance to Eretz Yisroel will be the reason for our inheriting Eretz Yisroel.
The Ohr Hachaim has another explanation for this.
The Torah tells us לֹא בְּצִּדְּקָתְּךָ וּבְּיֹשֶר לְּבָבְּךָ אַתָּה בָא לָרֶשֶת אֶת אַרְּ צָם כִּי בְּרִּ שְּעַת הַגּוֹיִּם הָאֵלֶה ה' א לוקיךָ מוֹרִּ ישָם מִּפָנֶיךָ וּלְּמַעַן הָקִּים אֶת הַדָבָר אֲשֶר נִּשְּבַע ה' א לוֹקיךָ לְּאַבְּרָהָם לְּיִּצְּחָק וּלְּיַעֲקֹב - Not for your righteousness, or for the uprightness of your heart, do you go in to possess their land; but for the wickedness of these nations Hashem your G-d drives them out from before you, and that He may establish the word which Hashem swore to your fathers, to Avraham, to Yitzchok, and to Yaakov.
Klal Yisroel’s life in Eretz Yisroel was not based on their own merits, and they were always in a precarious position. However, says the Ohr Hachaim, if Klal Yisroel follow this command of Hashem, they will merit Eretz Yisroel on their own accord. They will not need the wickedness of the other nations to be able to live in Eretz Yisroel.
The possuk now says that they shall write the words of the Torah on the Matzevah. With this, they will be able to enter the Land of Hashem, the land that He promised our ancestors, without using their merit or the wickedness of the other nations.
