Rashi explains that since man was created in the image of God, anything that disparages man is disparaging God as well.
Chilul Hashem, bringing disgrace to the Divine Name, is one of the greatest sins in the Torah. The opposite of chilul Hashem is kiddush Hashem, sanctifying the Divine Name. While this topic has several dimensions to it, there is a living kiddush Hashem which occurs when a Jew behaves in a manner that merits the respect and admiration of other people, who thereby respect the Torah of Israel.
What is chilul Hashem? One Talmudic author stated, "It is when I buy meat from the butcher and delay paying him" (Yoma 86a). To cause someone to say that a Torah scholar is anything less than scrupulous in meeting his obligations is to cause people to lose respect for the Torah.
Suppose someone offers us a business deal of questionable legality. Is the personal gain worth the possible dishonor that we bring not only upon ourselves, but on our nation? If our personal reputation is ours to handle in whatever way we please, shouldn't we handle the reputation of our nation and the God we represent with maximum care?
Jews have given so much, even their lives, for kiddush Hashem. Can we not forego a few dollars to avoid chilul Hashem?