fashioned it into a molten calf: The idol might have been a bull, similar to the Egyptian god Amon-Re, which the people knew from Egypt. The bull represented power, domination, and fertility. The writer might have said calf to despise the idol. Or, Aaron might have made a calf, thinking this sin was less severe than making a full-sized bull.
These, O Israel, are your gods, who brought you up out: The people credited the idol with what they had just said Moses did (32:1). Idolatry shows the belief that the divine and visible worlds are continuous. This view suggests one can access divine power by manipulating the god through the idol.
God insisted the opposite is true. God's creation cannot contain or limit him. No one can receive his blessing by manipulating creation, either in a ceremony or otherwise. God's blessings are for those who give up their own efforts to feel secure and come to him in the ways he has revealed.