In pleading with Hashem to be merciful towards His people and to forgive their iniquities, Dovid HaMelech says "...למה יאמרו הגוים," why should the world ask where Hashem is (תהילים ע"ט:י'). He thus makes a very important point to remind us about Chillul Hashem, desecrating Hashem's name. In a Posuk familiar to us from Tachanun, he states in the previous sentence "וכפר על חטאתינו למען שמך", and forgive our sins if only for Your name's sake (שם פסוק ט'). Dovid HaMelech then underscores, after we beg for forgiveness, that even if we are not deserving of it on our own, Hashem should forgive us anyway because "what will the world say" if we, the chosen people, are not forgiven.
This theme can also be seen in this week's Parsha. Moshe, in great desperation, turns to Hashem and asks how Paroh will ever listen to him if Bnai Yisrael did not listen to him, especially since he has a speech impediment (שמות ו:י"ב). Rashi, commenting on this Posuk, notes in the name of Chazal that this is one of the ten examples in the Torah of a Kal Vachomer (the transitive property of an inequality). It is only logical. If the Bnai Yisrael, who stand to be saved, won't listen to Moshe, how will Paroh, who stands to be punished, listen to Moshe? But what does the conclusion of Moshe's statement, stressing that he is handicapped with his speech, have to do with this question? What does this add?
The Sefer "על התורה", by Rabbi Mordechai HaKohen, offers the following brilliant interpretation. Moshe was actually thinking about something else entirely. He wondered: what will happen if the Jews will not listen to him but Paroh and the Egyptians will listen to him. What will happen if Paroh will let the Jews out and the Jews will not want to leave? What will happen if we are in an embarrassing situation, confronted by our failure to listen to and obey our own G-d? For this, Moshe realizes he will have no answer to Paroh, and thus he stresses that he is an ערל שפתים, with a speech handicap. He will have nothing to say.
Klal Yisrael must recognize not only its responsibility to Hashem as a collection of individuals, but as a nation, which is observed very closely by the whole world. We must also understand that we are our brothers' keepers, which means not only that we must be responsible for one another, but also realize what the world will say about the things we do. Imagine how many more people would be truly "righteous" rather than merely "religious" if they realized the Chillul Hashem that some of their actions caused. A "religious" veneer not only does not lessen the severity of the specific wrong that is being violated, and which is judged in its own right, but rather adds a dimension to our transgressions when we realize the far-reaching results of what we do in terms of the perception in the world at large.
Just as Moshe feared the great Chillul Hashem which would result from the failure of Bnai Yisrael to listen to him, Dovid HaMelech feared the Chillul Hashem which would result from our not being forgiven for our sins. Dovid HaMelech's fear is in fact highlighted by Chazal in our Shabbos davening, in Av Harachamim. We should think about this and heighten our awareness of the broader implications of the things we do and say.