הנה אנכי בא אליך בעב הענן בעבור ישמע העם בדברי עמך וגם בך יאמינו לעולם
Behold, I come to you in the thick cloud, that the people may hear when I speak with you, and believe you forever.The question that this verse raises is that not everyone has believed Moshe's words forever. Particularly in the modern era, there are many Jews who do not believe in the truth of the Torah. And if one includes in those who do not believe Moshe those Jews who decided to reject the commandments of the Torah and adopt either Christianity or Islam, then there have been many Jews throughout the past 2,000 years who have failed to believe in Moshe.
Click here to read moreA simple answer is that the promise is that there will always be some Jews who believe Moshe, and not necessarily that every Jew will always believe. However, the Rambam did not interpret the verse in that way. In his Iggeres Teiman (link), the Rambam writes that the descendants of any Jew who was at Mt. Sinai will always remain faithful to Judaism. Anyone who rejects Judaism proves that his ancestors were not at Mt. Sinai. This is also how his son explains the verse in his commentary to Exodus 19:9. If so, how do we understand the reality we see? Even the greatest families have lost members to non-observance and disbelief.
One answer could be that our community is largely descended from converts who did not physically stand at Mt. Sinai for the giving of the Torah, certainly a debatable suggestion. Another is offered by R. Yisrael Ya'akov Kanievsky in a letter to R. Moshe Mordechai Schlesinger and published in the latter's Mishmar Ha-Levi'im (as quoted in Talelei Oros, Ex. 19:9). R. Kanievsky suggests that the promise only relates to natural belief. The descendants of those who stood at Mt. Sinai will all naturally believe in Moshe. However, this does not prevent external influences--secular studies, newspapers, etc.--from drawing them away from this belief.
Alternately, one could suggest that this is somewhat of an exaggeration. The Gemara (Nedarim 20a) states about someone who lacks shame that it is clear that his ancestors did not stand at Mt. Sinai. And the Gemara (Beitzah 32b) states about someone who does not show mercy on creatures that it is clear that he is not descended from Avraham. Does the Gemara mean that literally or is it merely a generalization intended to teach a moral point? I think the latter, and that is probably what the Rambam intended as well.