The answer to this question is provided by one of the pastoral interns, Dave Nannery.
A Christian is a follower of Christ; someone who puts his or her faith in the Jesus Christ of biblical history and demonstrates this faith by a life dedicated to honoring Christ. The Bible is very clear that no one can be saved from sin and its punishment without first repenting from sin and placing his or her faith in Christ's atoning work on the cross. This work redeemed us from sin and death, bore the wrath of God for our sin, and reconciled us to the Father. God has chosen faith in Christ to be the basis for our justification (being declared righteous) before Him. Thus, salvation is by faith alone. In Romans 10, we see the apostle Paul declaring the necessity of faith. He begins by saying that "if you confess with your mouth that Jesus is Lord and believe in your heart that God raised him from the dead, you will be saved" (v. 9). As Pastor Whipple recently pointed out (in his sermon on Luke 12:8-12), a person with true faith will gladly confess Christ as Lord. Sadly, those who do not believe in Jesus Christ have rejected Him. Even if they have never heard of Him, they have rejected the evidence in creation which points to Him (Romans 1-2).
But what about passages such as Romans 5:18, which says that "as one trespass [by Adam] led to condemnation for all men, so one act of righteousness [by Jesus Christ] leads to justification and life for all men"? Doesn't this mean that Christ's death on the cross will save every person on earth? If so, this would undermine Paul's entire line of reasoning in the book of Romans! In fact, the context of the surrounding passage (particularly v. 17) explains the meaning of this verse: all people who are in Adam (all of humanity) have been condemned, while all people who are in Christ (all genuine Christians) are justified. Similarly, a handful of other scripture passages may, at first glance, seem to teach that people can be saved without faith in the historical Christ. However, these must be understood in the light of many more clear biblical statements that salvation is by faith alone (for example, John 3:36, Romans 3:21-26, Galatians 2:16, Ephesians 2:8-9).
We must understand that no one can be saved unless he or she first trusts in Jesus Christ for forgiveness. It is not enough to have a vague idea of who God is or simply to follow a moral code. Therefore, it is important for Christians to introduce others to the bad news of our sin against God and the good news of Jesus Christ's saving work. Without first hearing the gospel, no one can believe and be saved (Romans 10:14-17). Finally, we must carefully examine our own lives to see whether our behavior shows that we have placed our faith in Christ or in something or someone else. We cannot be satisfied with merely labeling ourselves as "Christians." We need to believe the truth of God's Word and demonstrate this faith with our actions (James 2:18).
Further resources:
Let the Nations Be Glad! by John Piper
Bible Doctrine by Wayne Grudem
"How Shall People Be Saved? Part 1" by John Piper