Christianity is a religion based upon the teachings and miracles of Jesus. Jesus is the Christ. The word, "christ," means anointed one. Christ is not Jesus' last name. Jesus is the anointed one from God the Father who came to this world, fulfilled the Old Testament laws and prophecies, died on the cross, and rose from the dead physically. He performed many miracles which were recorded in the Gospels by the eyewitnesses. He is divine in nature as well as human. Thus, He has two natures and is worthy of worship and prayer.
Christianity teaches that there is only one God in all existence and that God made the universe, the Earth, and created Adam and Eve. God created man in His image. This does not mean that God has a body of flesh and bones. Image means the likeness of God's character, rationality, etc. Because we are made in the image of God, every person is worthy of respect and honor. Furthermore, this means that we did not evolve through random processes from a single-celled organism into rational, emotional beings.
God created Adam and Eve and put them in the Garden of Eden and gave them the freedom to choose between right and wrong. They chose to sin. Sin is doing that which is contrary to the nature and will of God. For example, God cannot lie, therefore, lying is sin. The sin of disobeying God that Adam and Eve committed resulted in their being expelled from the Garden of Eden as well as suffering the effects of death.
As a result of their sin, their children and all of us inherited a sinful nature. In other words, our offspring are not perfect in nature. Think about it. We don't have to teach children to be selfish or how to lie. They know how to do these things naturally. This shows that we are born in a fallen state.
Christianity teaches that God is a Trinity (Father, Son, and Holy Spirit, not three Gods), Jesus Christ is the second person of the Trinity, Jesus died on the cross and rose from the dead physically and that all people are under the righteous judgment of God because all people have sinned against God. It teaches that Jesus is the only way to be saved from the coming judgment of God and that salvation is received by faith in the work of Christ on the cross and not by anything that we can do to please God.
Where all other religions in the world teach that we must do some sort of good in cooperation with God in order to achieve the right to be in God's presence, Christianity is the only religion that teaches salvation by grace. This means that we are not made right before God by our efforts, sincerity, or works. Instead, we are made right before God by faith in what Christ did on the cross.
Christianity further teaches that once a person is "born again" (becomes saved), the Holy Spirit lives in that person and the person is changed: "Therefore if any man is in Christ, he is a new creature; the old things passed away; behold, new things have come," (2 Cor. 5:17). This means that God actually lives in the person, and the Christian then experiences a true and living relationship with God.
Therefore, "What is Christianity?" is best answered by saying that it is a relationship with the true and living God through the person of Jesus Christ by whom we are forgiven of our sins and escape the righteous judgment of God.