This is the second movie in John Carpenter's loose collection called The Apocalypse Trilogy (see The Thing and In the Mouth of Madness). This particular entry focuses on a group of researchers and a priest who have found living physical embodiment of Satan on Earth in the form of a green liquid trapped in a cylinder. As the group learns more about it, the liquid seeps out and possesses people. A group of homeless people barricade the church the team is in so they cannot escape. Eventually, Satan gets out and attempts to bring something called The Anti-God to Earth. Can this rag tag group stop Satan himself? You'll have to watch.
While there are some good creepy scares in this movie, it feels a little like sci-fi horror more than pure horror. Carpenter tries very hard to explain the existence of Satan in scientific terms and ground the action in a secular reality. It reminds me a bit of how Marvel insists Thor is an extra-dimensional being and not an actual god. At any rate, most people remember this because of Alice Cooper as the leader of the homeless human barricade who murders anyone trying to escape the church where all the action takes place. Besides that, you get a decent turn from Donald Pleasence as the preacher who is trying to Mulder up all these Scullys he has invited into his church.
The effects don't age so well but this is still a solid 80s horror flick. I would give it a C+.