On a very basic level, a true Christian is simply someone who believes that Jesus rose from the dead and who has made Him Lord of their life (Romans 10:9), but in practice being a Christian means much more than that.
The first step to becoming a true Christian is to realize that we have all sinned and done things that are wrong (Romans 3:23). Unfortunately for us, any sin, from a white lie to mass murder, is unacceptable to God because He is Holy and sinless. Each of us is living under a death sentence because of our sins (Romans 3:23). This death refers to an eternal punishment.
Fortunately, God provided a way out. God first sent His Word in the form of the books that make up the Bible. They told people to expect a Messiah (Savior) who would save them from their sin-induced death penalty. Next, God sent His Son, Jesus, to take human form and take our punishment for us (Romans 6:23, 5:8).
Now it is up to us. God has offered His forgiveness. We only have to accept it. We must believe that Jesus rose from the dead and make Him ruler of our lives (Romans 10:9-13). Becoming a true Christian does not stop there, however. Our faith in Christ saves us, but our faith must lead to action. Faith without action is useless (James 2:17). What kind of action should Christians take?
There are several paths of action that true Christians should take. The most important is following the Great Commission (Matthew 28:18-20). Non-Christians who resent it when people tell them about Jesus commonly misunderstand this doctrine.
Christians have the one truth about spirituality and life. Jesus said, "I am the way, the truth, and the life. No one comes to the Father except through me" (John 14:6). Jesus commanded us to spread this gospel (good news) throughout the world. If people do not the truth, then their souls are in danger of Hell.
The important thing to remember about spreading the gospel is that people cannot be saved by force. Christianity is a religion of free will. People have to choose to accept or reject Christ. Failing to choose means a choice to reject. The Christian's mission is merely to inform people of the truth and let them make up their own minds.
Evangelism, the spreading of the gospel, is the most important action of a Christian for two reasons. First, Jesus specifically commands it. Second, it is a matter of eternal life or death.
As a second course of action, Christianity has a long tradition of helping the poor and sick. This tradition goes back to Jesus' day and before. Its roots lie in the fact that Christianity teaches that all life is valuable and should be treated with respect and dignity.
Christians are responsible for many of the great charities of the world. The Salvation Army and the Red Cross both have Christian roots. Christian relief agencies provide food for the hungry and medicine for the poor around the world.
A third course of action is the most controversial today. Christians have a duty to affect positive changes in the world. Throughout history this has been done several ways. Christians in scientific fields have given the world many technological advances. Indeed, modern science can be said to have Christian roots due to the fact that Christians were the first to combine scholastic observations with practical engineering to effect change on the environment.
Christians are also responsible for making education accessible to the masses. Many of today's colleges and universities started out as Christian seminaries. Harvard and Yale are two examples of formerly Christian schools.
Christian political activism has also changed the world for the better. The abolition of slavery was largely the result of Christians who believed that all men are created in God's image. The Civil Rights movement of the 1960s was also largely the work of Christian activists. Even the foundation of the United States was made possible by the belief that all men are equal at the foot of the cross. This belief allowed political philosophers to question the divine right of kings and pave the way for democracy.
Today, Christian activists are fighting multiple battles. Christians are being persecuted around the world from Darfur to Iraq to Burma. The Christian belief in the sanctity of life leads us to oppose the killing of unborn babies. The Christian belief in the importance of stable families leads us to protect the sanctity of marriage.
In general, the US and the world are moving away from a moralistic culture to one of moral relativism. As Clarence Thomas put it, people believe that you can choose right and wrong instead of choosing between right and wrong. This leads to a situation where everyone does what is right in his own eyes. This leads to anarchy and a breakdown of society. It also means that Christians have their work cut out for them.
In summary, a true Christian is one who has faith in Jesus and follows the commandments of Jesus. Faith is the key to salvation, but to be a true Christian, your faith must lead to action.