Ok so when people worship an entity they send out power to grow and fuel that entity. So when the greeks and romans after them worshiped the greek gods, they grew strong. the greek pantheon, as a result, is one of the most powerful sets of gods.
Sadly over the years they lost some amount of power because not many people worshiped them and they became myth, but with many of us today believe and worship some form of god and normally they come from the greek pantheon.
but that also goes for every other god thats been worshipped in mass before. Belief is what makes a diety real. True belief, anyways. When someone believes in a diety so purely that their belife in them takes a hold on how they live their life the diety is suddenly a real thing...at least to them.