(For my MCCC philosophy students.)
The Kalam Cosmological Argument, as formulated by William Lane Craig is an evidential argument. If someone says "There's no evidence for God's existence" that is simply false. Using the evidence of a temporal universe, and then reasoning logically using that fact as a premise, one concludes (deductively) that our universe has a cause.
But why must that cause be God?
I was answering that question for my MCCC philosophy students after a recent class. This is abstract, logical thinking that is hard to grasp for students who are not used to critical thinking like this. I told them I'm going to make a post to help explain this further. I'm going to quote from Craig's excellent On Guard. He writes:
"The cause of the universe must therefore be a transcendent cause beyond the universe. This cause must be itself uncaused because we've seen that an infinite series of causes is impossible [using, e.g., German mathematician David Hilbert's reasoning that an actual infinite is impossible]. It is therefore the Uncreated First Cause. It must transcend space and time, since it created space and time [remember: in the "singularity" there is neither space nor time, and thus the laws of physics do not apply]. Therefore, it must be immaterial and nonphysical [logically so]. It must be unimaginably powerful, since it created all matter and energy." (99)
But why must the cause of the universe be a personal agent? Because if the cause of the universe is timeless and impersonal this cannot produce a temporal effect with a beginning like the universe. Craig writes:
"If a cause is sufficient to produce its effect, then if the cause is there, the effect must be there, too. For example, water freezes when the temperature is below 0 degrees centigrade; the cause of the freezing is the temperature's falling to 0 degrees. If the temperature has always been below 0 degrees, then any water around would be frozen from eternity. It would be impossible for water to begin to freeze just a finite time ago. Now the cause of the universe is permanently there, since it is timeless. So why isn't the unvierse permanently there as well? Why did the universe begin to exist only 13.7 billion years ago? Why isn't it as permanent as its cause?" (100)
By inference to the best explanation, a timeless, personal cause best explains the effect of a temporal universe. "The answer to this problem must be that the cause is a personal being with freedom of the will. His creating the universe is a free that is independent of any prior conditions. So his act of creating can be something spontaneous and new. This, we're brought not merely to a transcendent cause of the universe but to its Personal Creator." (Ib.)
Finally, Craig concludes: "The kalam cosmological argument gives us powerful grounds for believing in the existence of a beginningless, uncaused, timeless, spaceless, changeless, immaterial, enormously powerful, Personal Creator of the universe." (Ib.)