There is no sin more serious than the sin of idolatry. Paul is referring to the infamous incident of the golden calf (see Exodus 32:6). Although Aaron made one golden calf, claiming that this calf represented the Lord (Exodus 32:11), the actual request of the Israelites had been that he would make them gods
(Exodus 32:1). Their intention was clearly idolatrous, as is the intention of those who eat meat at a pagan temple.
7 Do not be idolaters as some of them were; as it is written, “The people sat down to eat and drink and rose up to play.”