NCU. William Lane Craig's argument is below, and my refutation follows:
P1. Everything that exists has an explanation of its existence (either in its own nature or in an external cause)
P2. The universe exists.
P3. If the universe has an explanation of its existence, that explanation is god.
C. Therefore the explanation of the universe is god.
Craig believes he has formulated a clever argument that will not fall prey to the usual cosmological counter--but it does. Adding the phrase "either in its own nature or in an external cause" does not help. Let's see how he intended to use this loophole. Adding the phrase "in its own nature" is an effort to fend off the question and infinite regress of, "What explanation can be offered to explain god?" The answer would be, "Another god." But that leads to an infinite regress, where the explanation of gods existence is explained by another god, and so on to infinity. Craig wants to use the notion that the explanation of god is "in his own nature" to prevent the infinite regress. However, this move by Craig provides a counter to his own argument. We can use the same explanation for the explanation for the existence of the universe, in that, it is "in its own nature." Let me illustrate this for you.
One could say, "Well, god exists, therefore he must have an explanation." Then Craig could simply answer that God's explanation is "in God's own nature." Note, this is merely by description, and assertion, without any facts or evidence. Worse, we can say the same thing for the universe, but in this case, one explanation has been offered by Lawrence Krauss using Quantum Mechanics. Whether this is true or not is neither here nor there, but the fact of the matter is that we have more evidence for the universe being explained by "its own nature" than we do for any "gods or goddesses" as the explanation for its existence. Furthermore, the universe is considered circular and infinite by many religions; for example, Taoism, Buddhism and ancient African religions; and this is supported by the Second Law of Conservation of Energy, as well as Quantum Theory, as pointed out by Dr. Krauss.
So we see from examining P1, that Craig commits a fallacy worse than the fallacy of "False Dilemma," for, in P1, he gives a disjunction, with two disjuncts. One disjunct being "in its own nature" and the other disjunct is "an external cause." He goes on to P3, and limits the possible explanations of the universe ONLY being an external cause. There is no justification for such a limitation, as I have already shown.
Furthermore, I do not have to provide an explanation of how the explanation of the universe is "in its own nature." As Craig himself points out about the explanation of a "best explanation":
"...that can be left open for future inquiry... If the best explanation needs an explanation, we're left with an infinite regress."
Therefore, I do not have to provide an "explanation of my explanation" of the universe being "in its own nature." Clearly, this explanation is just as viable, if not more viable, and a better explanation, (due to scientific inquiry) than saying "Goddidit!!!" This shows that P3 is not justified, and the argument does not work, as it is an open question as to the explanation of the existence of the universe. William Lane Craig's ATTEMPT to prove god's existence as the best explanation for the existence of the universe in the above argument fails.