Theism is the study of God. The belief in a God. Monotheism is the belief in one God. Polytheism in many Gods. Atheism in no Gods, and Pantheism is the belief that in all things is God.
This is not necessarily the same thing as saying everything is God, but in everything there is God. We have the power of God within ourselves and in everything there is this presence. This makes all things connected in a very special way. For everything is, when summed down, all the same essence. Star dust. God. The One.
Many cosmic humanists believe in what Socrates would probably call the form of God, or the ideal of God that exists out there somewhere in the cosmos. The perfect form of God and from this perfect form everything was created and to this perfect form everything will return.
Think of it in analogy to an atom. The physical world is made up of atomic particles and molecules. There is nothing physical that does not have an atomic make-up. In the same way, everything was made out of this cosmic form, so everything has a part of God in it, for it was formed from it. (Though perhaps God is not the right word for the ideal). The purpose, as many cosmic humanists see it, is to be reintegrated into this perfect form. They have different names for this perfect form, The One, Enlightenment, Brahman, etc. but it is all encapsulated in this idea of Pantheism.
Some, more than others, believe an implication of this is that we have the power of God, only we have forgotten how to use it, we are formed and are in essence this perfect form but have somehow misplaced the true knowledge to be able to connect back to the power and knowledge of the perfect form. Trapped within our physical constructs and too encapsulated by our own realities created by our own imaginations we have forgotten and must seek enlightenment to re-obtain that true knowledge of God.
Pantheism has many faces and many factors that make one belief different from another, here I attempted to describe the principle without pidgeon-toeing the belief by over-simplifying or over-exaggerating the ideal. I hope it has made sense.
Christianity believes that God is omnipresent, meaning He is present everywhere, His presence is ever present. There is nowhere He cannot be and there is nowhere He is limited to. The difference is that God’s presence in Christianity is distinct from ourselves, and all other things. God may rest His presence within us if He so chooses, but that does not make us God nor does it necessitate our ability to use God’s power, but He is distinct in and of Himself. Out of God’s power everything was created, but in Christianity, that which exists is distinct from God, even though we too believe that nothing exists outside of God’s sustaining power.