In a world where a "god" constitutes nothing more nor less than a critical mass of belief in a particular concept or ideal with a name plastered onto it. The gods are magical in nature being created by the magic energy derived from those who believe but the belief is the corner stone. Gods, when they exist at all, are physically invulnerable.

Given this origin and definition how would someone, human, living in this world go about killing such a being?

When answering assume a world with medieval levels of technology and universal but not uniform religion, magic is not a thing that happens in every day life but miracles are relatively common AKA magical displays are the exclusive province of gods, not men.

Please note this is about an in-world solution not an author's solution to the problem at the havewaivium level.