They had constrained Aaron to make a golden calf and were worshipping and offering sacrifices to it. Besides violating the commandments concerning worship the people violated those concerning morality and sat down to eat and drink and rose up to play. Aaron alibied and said to Moses, thou knowest the hearts of the people, that they are set on mischief. Their hearts were set on idolatry.
Men have a strong tendency to worship idols.
When we view the history of the ancient world, we find people worshipped idols everywhere. Among the ancient nations Israel is distinguished for her prohibition of images in worship. The prophets denounced idols. Though they had been taught differently, the people of Israel still worshipped idols until they were carried captive into Babylon. Only by living through that terrible experience were they purged of image worship.