Rashi says: An image of Moshe was shown to them by the Satan as Moshe was being carried in the sky.
Rashi explains how the Satan was able to deceive the Jewish people into believing that Moshe had died. The Daas Zekainim states that there were three groups involved in the incident of the Golden Calf. One group intended only to create a leader to replace Moshe but not to make an idol of the calf. A second group accepted the Golden Calf as an idol. The third group consisted of the members of the tribe of Levi, who remained resolute in their belief in Hashem, never for one moment giving any credibility to the Golden Calf. Their exemplary display of faith in Hashem is attributed to that they were the direct recipients of the Torah from Yaakov, who in turn, received if from Avrohom and Yitzchok. Their incessant study of Torah protected them from falling into the pitfalls of doubt and uncertainty. The Chovas Halevovos writes that the method employed by the yetzer hora (evil inclination) to ensnare one into committing a sin is deception, clouding the truth with a veil of illusion and camouflage. It is only through the constant study of Torah that a person develops the clarity of vision to pierce through these various disguises and deceptions to see the truth. The tribe of Levi did not falter in their study of Torah, and therefore did not fall prey to the cunning of the Satan. It was their strong attachment to Hashem that gave them the courage to withstand the pressure brought upon them by their misguided brethren. It is only through the diligent study of Torah that we can face the uncertainties of daily life, finding courage in our belief in Hashem.
