This probably sounds like a ridiculous question, but in all seriousness, many Jews lost their lives in the aftermath of the worship of the Golden Calf (Shemos 32:27-28). While I understand only 40 days or so before they began to worship the Calf, G-d had spoken to them and instructed them not to worship any other G-d. How could they be sure that their mistaken belief in some sort of incarnated or intermediary, or a physical form of G-d was in fact false, to the extent that they deserved death?
They knew it wasn't God from the start:
וַיַּרְא הָעָם, כִּי-בֹשֵׁשׁ מֹשֶׁה לָרֶדֶת מִן-הָהָר; וַיִּקָּהֵל הָעָם עַל-אַהֲרֹן, וַיֹּאמְרוּ אֵלָיו קוּם עֲשֵׂה-לָנוּ אֱלֹהִים אֲשֶׁר יֵלְכוּ לְפָנֵינוּ--כִּי-זֶה מֹשֶׁה הָאִישׁ אֲשֶׁר הֶעֱלָנוּ מֵאֶרֶץ מִצְרַיִם, לֹא יָדַעְנוּ מֶה-הָיָה לוֹ.
And when the people saw that Moses delayed to come down from the mount, the people gathered themselves together unto Aaron, and said unto him: 'Up, 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.' (Shemot 32:1)
They weren't fooled by some "god" that was presented to them; they made it, knowing what they were doing. They said to Aharon "we don't know what's up with that other one; make us a new god".
They were told 40 days earlier not to make idols or chase after other gods. They did it anyway. They might have been frightened, but the p'shat is that they were not unaware -- they were, indeed, quite intentional in their actions.