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Christians are often called "believers," while non-Christians are termed "unbelievers." Scripture itself speaks this way: we read that "believers were the more added to the Lord" (Acts 5:14), and that they should not be "unequally yoked together with unbelievers" (2 Cor. 6:14). There is obviously two classes of people distinguished by whether they believe or not. It can rightly be said that what separates Christians from non-Christians is the matter of faith.
Christians believe certain things which non-Christians do not. Christians believe the claims of Christ and the teachings of the Bible to be true, but non-Christians disbelieve them. Christians have faith in Christ and trust His promises; non-Christians do not believe in Him and doubt His word. It is quite natural, then, that the gospel can be called "the word of faith" (Rom. 10:8). Becoming a Christian entails that you "believe in your heart that God raised Him [Christ] from the dead" (10:9); likewise, "he who comes to God must believe that He is, and that He is the rewarder of them who diligently seek Him" (Heb. 11:6). Examples could be multiplied. What sets Christians off from non-Christians is the matter of belief or faith.
The difference, however, between them is more than that in an important sense, and we need to understand this if we are going to do a faithful job in defending the faith. The Christian claims to "believe" the teachings of Scripture or to have "faith" in the person of Christ because the element of trust is so prominent in our relationship with the Savior. But the Christian actually claims more than simply to believe Christ's claims to be true. The Christian also affirms that he or she "knows" those claims to be true. What is involved in saving faith is more than hope (although that is present) and more than a commitment of will (although that too is present). Job confidently asserted, "I know my Redeemer lives" (Job 19:25). John indicated that he wrote his first epistle so that those "who believe on the name of the Son of God" "may know that you have eternal life" (1 John 5:13). Paul declared that God "has furnished proof" that Jesus will judge the world (Acts 17:31). Jesus promised His disciples that they would "know the truth, and the truth shall set you free" (John 8:32).
In what way does knowledge go beyond belief? Knowledge includes having justification or good reason to support whatever it is you believe. Imagine that I believe there are thirty-seven square miles in a particular city, and imagine also that it just so happens that this claim is accurate—but imagine as well that I simply got this answer by guessing (rather than doing measurements, mathematics or checking an almanac, etc.). I believed something which happened to be true, but we would not say that I had "knowledge" in this case because I had no justification for what I believed. When we claim to know that something is true, we are thereby claiming to have adequate evidence, proof or good reason for it.
The difference between the Christian and the non-Christian is not simply that one believes the Bible and the other does not. People's beliefs can be frivolous, random, or silly. The Christian also claims that there is justification for believing what the Bible says. The non-Christian says, to the contrary, that there is no justification (or adequate justification) for believing the Bible's claims—or, in stronger cases, says that there is justification for disbelieving the Bible's claims. Apologetics amounts to an inquiry into and debate over who is correct on this matter. It involves giving reasons, offering refutations, and answering objections.