Pantheism is the belief that God is everything and everything is God.
Pantheism is the belief that God and the Universe are identical, and that there is no difference between God and the World. One rationale for pantheism, as opposed to monotheism, is that, for the human practical sense of the physical world, there seems no possibility of God's omnipresence in the Creation unless God is identical to the Creation. Pantheism therefore holds all things to be divine; but, this view was rejected by Saint Augustine because this view would mean that even sin was divine. Historically some philosophers, such as John Toland, have used pantheism to mean the equivalent of atheism, denying that the world is the product of divine creation, denying any guidance of God, and even denying the existence of sin. Or alternatively, by saying everything is divine, ultimately, nothing is. This can be demonstrated with the super heroes metaphor: If you assume a place where everyone has special powers, no one is special, hence, no superheroes.
Some pantheistic ideas were bandied about as early as the time of the ancient Greek philosophers, who felt that everything in the Universe was made from the same divine substance. Pantheism resurfaced in the 1700s from the writing of Benedict Spinoza, who laid out certain philosophical justifications for the idea. Two refinements of pantheism developed in the 1800s, panentheism and pandeism. Panentheism tried to bring pantheism back into monotheism by describing the Universe as one part of God existing at the same time as a transcendent part of God existed apart from it. Pandeism tried to combine pantheism with deism, which was then at the height of its popularity, by describing a process in which God became the Universe and in that process ceased to act as God.
Modernly, pantheism has declined from the level of popularity achieved in Spinoza's day.