If a stranger lives as a foreigner with you in your land, you shall not mistreat him.
The Ohr Hachaim notices that the Torah changes the number in the possuk. First, the singular expression is used – אִּתְךָ, and then the Torah switches to the plural expression – בְאַּרְ צְכֶם.
The Ohr Hachaim explains that all converts to Judaism are attracted to Torah through the first convert who would teach others to join him. This was Avraham Avinu. Another method of causing gentiles to wish to join Klal Yisroel is through Eretz Yisroel. Eretz Yisroel is a holy land, and it attracts the sparks of holiness from the entire universe. Those sparks had been captured by the forces of evil, trapped under their influence, and needed to be redeemed. The power of the Holy Land draws them in, removing the dross and influences of captivity, and bringing them under the wings of holiness.
Both of these ideas are alluded to in this possuk. First, the convert lives with you (sing.), which refers to the singular Tzadik of the generations who had the ability to reveal and polish these latent sparks of holiness in the souls of gentiles. The convert could also be in your (pl.) land, which refers to Eretz Yisroel's ability to draw converts to Judaism.
Another explanation offered by the Ohr Hachaim is that the convert, or stranger, is the Shechina. This world is a physical one, and the spiritual Shechina is a stranger in this world. However, Hashem wished to create a dwelling place for Himself in this world, as the possuk says אֺהֶל שִּׁכֵּן בָאָדָם – a tent which He established among men. Hashem dwells with each person, but that is in the merit of the Holy Land, where Hashem prepared a place for Himself to dwell. Hashem leaves His Heavens and comes to dwell amongst us.
When this happens, we are exhorted לֺא תוֹנוּ אֺתוֹ - do not mistreat Him. We may not trick Him in any way. This refers to the worst kind of chicanery; when a person attempts to trick Hashem through insincere worship and misusing His Mitzvos.
