So far, "On Guard" has laid the important groundwork for understanding the pitfalls atheism faces right out of the gate. Not only does atheism fail to produce meaning, purpose, and values (see post "Absurdity of Life Without God"), but it also falls short in the areas of philosophy and reason, as atheism fails to produce an explanation of the universe - unless "inexplicability" is considered a good explanation (see post "Why Does Anything At All Exist?"). At the same time, Craig spent the previous chapter showing how God is a viable explanation for the universe, God maintains the explanatory property of necessity, and is the only explanation with explanatory power apart from entities like abstract objects, which are not viable options for reasons addressed elsewhere.
In this next chapter, "Why Did the Universe Begin?" Craig begins to move beyond abstract philosophy, and starts to bring in some of the scientific evidence we find in the universe. It is an important chapter for understanding the more abstract ideas laid out in the previous chapter, and it helps the reader to really begin understanding the weight of the dilemma atheists face. It also helps the reader to understand the tremendous evidence for a being outside of the universe. It still does not point us to the Christian God, but we are moving more and more in that direction, as Craig continues building the positive case for God.
Basic Argument: (also found here)
1. Everything that exists has an explanation of its existence [either in necessity of it's own nature, or an external cause].
2. If the universe has an explanation, that explanation is God.
3. The universe exists.
4. Since the universe exists, it must have an explanation of its existence.
5. That explanation is God.
The argument above is unarguably logically valid. IF the premises are true, the conclusion MUST be true. Obviously, atheists will not agree with the conclusion, so they must disarm the argument by disproving at least one of the premises 1-3. Therefore, Craig spends most of his chapter defending the premises.