"On Guard" begins with a foreword by former skeptic turned apologist Lee Strobel in which he praises Dr. Craig as "among the very best defenders of Christianity of this generation" and then recounts several of Craig's debates with atheists.
In the first such instance, writes Strobel, an atheist organization proposed a debate pitting one of their foremost atheists, Frank Zindler, against the best apologist Christians could muster, and the Christians put forth Bill Craig who debated Zindler before a packed church auditorium and massive media coverage in 1993, and, according to Strobel, Craig trounced Zindler, which led to several Christian conversions right on the spot and shocked the atheist community which had anticipated that they could easily debunk Christian teachings.
In another famous and much more recent debate, Dr. Craig debated the renowned polemicist writer and debater Christopher Hitchens and, according to this atheist, "spanked Hitchens like a foolish child."
Strobel then proceeds to explain that Dr. Craig, in his debate with Zindler, posed five essential arguments for the existence of the Christian God:
"First, the beginning of the universe clearly points toward a Creator (“Whatever begins to exist has a cause; the universe began to exist; therefore, the universe has a cause”). Second, the universe’s incredible fine-tuning defies coincidence and exhibits the handiwork of an intelligent designer. Third, our objective moral values are evidence that there is a God, since only He could establish a universal standard of right and wrong. Fourth, the historical evidence for the resurrection—including the empty tomb, eyewitness accounts, and the origin of the Christian faith—establish the divinity of Jesus. And, fifth, God can be immediately known and experienced by those who seek Him."
Next, I will summarize Dr. Craig's key points in Chapter 1: "What Is Apologetics?"