Why would a pope threaten to excommunicate a king?

Popes used the threat of excommunication, or banishment from the Church, to wield power over political rulers. For example, a disobedient king's quarrel with a pope might result in excommunication. This meant the king would be denied salvation. Excommunication also freed all the king's vassals from their duties to him.

congress also has implied powers, which derive from the necessary and proper clause of the constitution and permit congress "to make all laws which shall be necessary and proper for carrying into execution the foregoing powers, and all other powers vested by this constitution in the government of the united states.