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 Ne devenez point idolâtres, comme quelques-uns d'entre eux, selon qu'il est écrit: Le peuple s'assit pour manger et pour boire; puis ils se levèrent pour se divertir.