Ki Tisa contains some of the more baffling events in the Torah as the Jewish people, shortly after hearing the ten commands, proceed to create an idol and worship it. Various explanations have been given for what happened but I think it is instructive to explore the mindset behind these actions.
It is Moshe's failure to return on time that encourages the construction of an idol to take the place of Moshe saying, "Rise and make us a god who shall go before us; for as for this Moses, the man that brought us up out of the land of Egypt, we know not what is become of him." The statement contains two admissions, that Moshe is a man and yet that he brought them out of Egypt.
If they simply wanted another man to take Moshe's place, they would have elected a leader. Clearly while they believed that Moshe was a man, they did not believe that he was merely a man. At the Sea, it is said, "Vayaminu Be'Hashem U'Be'Moshe Avdo", "They Believed in G-d and in Moshe His Servant". When Moshe first set out to reveal his prophecy to the Jews, he brought signs to cause the Jews to believe in him. Then with many of the plagues, Moshe or Aaron took an action followed by a miracle. It is not unreasonable that some had come to believe in Moshe, but not quite in G-d. While Moshe repeatedly spoke of G-d, what people hear is not the same thing as what they are told.
At the sea things changed, for Moshe began to pray and was cut off. Instead the miracle came as the Jews advanced into the sea. Their own reliance on G-d brought the miracle and afterward, they believed in G-d and Moshe only as his servant. Yet at Sinai they heard G-d and were unable to stand it and instead chose to receive the commandments from Moshe. As the lawgiver and with his departure, Moshe came to seem coequal with G-d or an aspect of G-d.
The phrasing of the demand for an idol is telling, "כִּי-זֶה מֹשֶׁה הָאִישׁ אֲשֶׁר הֶעֱלָנוּ מֵאֶרֶץ מִצְרַיִם, לֹא יָדַעְנוּ מֶה-הָיָה לוֹ" "For this Moshe, this man who led us out of Egypt, we do not know what has happened to him." The emphasis is on Moshe as only a man, a form that was apparently in their minds as inadequate to contain divinity. So they replaced him with a more supernatural form, gold in place of flesh, animal in place of man.
How do we know that the worship of Moshe himself was a problem? For one thing, his tomb was hidden, fairly unique when the tombs of great men even predating him are accessible. Secondly Moshe was never allowed to enter Israel. The presumed sin for which he was forbidden to enter the land involved a repetition of a miracle involving bringing water from a rock. The first time Moshe struck the rock while the second time he was commanded to speak rather than strike it. What difference does this make?
Moshe was chosen as the most humble of all men. Why in particular was humility a choice virtue? Why not one of his other positive traits? To be humble means to displace yourself in favor of your ego and Moshe had been chosen to perform the greatest miracles in history, feats that could easily be construed to demonstrate his own divinity. As the humblest man, Moshe was to be the least plausible candidate for being construed as a deity, let alone presenting himself as one.
In striking a rock, Moshe was functioning as superior to the rock. Yet by being told to speak to it the second time around, Moshe was to demonstrate that even when dealing with a rock, he would show a humility so great as to address it, rather than to hit it, so as not to set himself above even the rock.
Many years earlier, Moshe had carved the second set of commandments out of rock and shattered the first, in response to an idolatry that had been rooted in the perception of him as a deity. Yet the rock that he struck the second time was a failure to set aside that humility and for this he would not lead the second generation that had not known the calf into the land.
After the events of the second tablets, the light from Moshe's face shone so brightly that he was forced to wear a mask except when he taught, for then the light of divine inspiration could clearly be seen, reflecting the divine light on his face. By contrast when facing people who so easily might fall into thinking of objects and men as gods, he instead wore a mask to cover the light.