Pantheism is the position that God and nature are the same thing. Pantheism comes from two Greek words, pan meaning "all" and theos meaning "god." So, it would teach that all the stars, galaxies, planets, mountains, wind, and rain are all one and the same . . . part of what God is. So, pantheists would say that all is God.
Biblical Christianity teaches that God is separate from His creation, and He created it (Gen. 1:1-30) where pantheism says that God and creation share the same nature and essence.
A huge problem with pantheism is that it cannot account for the existence of the universe. The universe is not infinitely old. It had a beginning. This would mean that God also had a beginning, but how can something bring itself into existence? This is impossible, so this leaves us with the question of where God and the universe came from. Pantheism cannot answer this question, and it naturally leads to absurdities.