Christian apologetics is the discipline or practice of defending and commending Christianity. Christianity as a worldview competes with a host of other worldviews to accurately represent things as they are. Imagine with me a Christian engaging a non-Christian in apologetics. By what criteria will he judge the arguments? Ah, but here is the kicker: The debate is about the criteria themselves.
How so? When a Christian engages a non-Christian, each makes a claim about ultimate reality—the way things really are. Now the way things really are affects the way people can know things. (Philosophy says that your ontology [philosophy of what is] has implications for your epistemology [philosophy of how we know what is].) The Christian derives his ontology and epistemology from biblical and systematic theology; the non-Christian derives his from somewhere else—if an atheist, perhaps from his own experience filtered through his own reason. The Christian and the non-Christian, because they have different ontologies and epistemologies, hold very different ideas about what is scientifically possible, morally just, or rationally plausible. (For instance, the vicarious atonement is morally repugnant to unbelievers, cf. 1 Cor. 1:18–24.) Worldviews clash over ultimate issues, including what categories best sort data and what criteria best judge arguments. Christianity tells us that even more is at stake—namely, how we may be right with God.