Rabbi Moshe of Coucy (d. 1260 C.E.): Sefer Mitzvos Gadol, Negative Commandment 74:

Israel must separate themselves from the vanities of the world and hold onto the seal of the Holy One, blessed is He, which is truth. They should not lie to anyone, Jew or non-Jew, and they should not deceive anyone in any manner, as the prophet says, "The remainder of Israel will do no wrong and will not speak falsely, and there will not be found in their mouths deceptive speech" (Zephaniah 3:13).

Soon the Holy One, blessed is He, will come to redeem the Jewish people. If the Jewish people acts honestly, the gentiles will say, "G-d acted fairly when he picked such people of truth to be His chosen nation. The Torah in their mouths is true." But if they act dishonestly, the gentiles will say, "Why did G-d choose for His portion thieves and swindlers?"

Also, the prophet (Hoshea 2:25) says, "And I will plant them for Myself in the earth," and the Talmud (Pesachim 87b) comments, "A person only plants one kor in order to reap many korim. Here too, G-d scattered the Jewish people among the nations so that converts would join them." If the Jews deal dishonestly, who will join them?