The Argument from Contingency
The Argument from Contingency is one of the strongest arguments for the existence of God. The most basic form is as follows:
1. If something exists, there in turn must exist a cause for that thing to exist.
2. The universe-all beings and things in space and time-exist.
3. Therefore, there must exist a cause for the existence of the universe.
4. The cause of the existence of the universe cannot exist within the universe or be limited by time and space.
5. Therefore, the cause of the existence of the universe must transcend time and space.
If the critic seeks to deny premise #1, this would be an absurdity, since it would mean the universe is eternally self existent, which is refuted by science, including such principles as the 2nd Law of Thermodynamics, and the expansion of the universe.
The argument also mentions "all beings and things in time and space", as part of premise #2. You are one of those beings. You exist as a composite being of matter and spirit. That material aspect of yourself is finite, limited, and changing. You rely on the existence of other beings and things for your existence. As an infant in the womb you relied on your mother for sustenance. As a child you relied on your parents for food, clothing, protection, shelter, etc. Today you rely on many things as well-food, oxygen, water, etc. This is what it means to be a contingent being. You exist if something else exists, and only exist as an effect from a cause.
However, this implies there must exist something that does not need other things for its existence. It is a necessary being, since all contingent beings logically rely on something to give them existence. So there must be something that does not exist conditionally; something which exists in and of itself. Unlike us, as composite beings of matter and spirit, this necessary being has no distinction of "parts", and does not change in space and time. This means the cause of all existence cannot be the universe itself or even somehow a part of the universe. It must be transcendent.