The Holy Trinity: This can be a confusing concept at first. While there is just one God, Christians believe that this God can take three different forms: God the Parent, similar to the Old Testament God; Jesus Christ, who is also thought of as the Son of God; and the Holy Spirit, the active presence of God in human life. These three parts of God have distinct features and purposes, yet all three are part of one God. To visualize this, one may think of the common molecule H2O. It can take three different forms—ice, water, and steam—yet these three forms are all H2O, since they share the same molecular structure.
In addition, the idea of faith is very important to Christians. While one should always act by the Golden Rule ("Do unto others as you would have them do unto you"), it is not by good deeds or moral behavior alone that one can go to heaven (a place of eternal spiritual life) after death. Christ said, "I am the way, the truth, and the life; no one gets to [God] except through me." Many Christians believe that this means that we cannot go to heaven without faith that Christ is the messiah sent by God to save us from our sinful natures. Traditional Christianity teaches that the world will come to an end, and that Christ will come a second time to judge the living and the dead on Judgment Day.
Development of Christianity
After the Ascension and Pentecost, Christ's disciples dedicated their lives to spreading Christ's teachings. The disciples Matthew, Mark, Luke, and John each wrote a gospel (literally meaning "good news") that detailed the life and parables of Christ. Disciples Paul and Peter later compiled the letters they had written to their converts, which stress the idea of salvation through faith in Christ. These gospels and letters, along with the book of Acts (which details the lives of the disciples) and the book of Revelation (which describes Judgment Day, Christ’s return to earth, and descriptions of heaven and hell), compose the majority of the New Testament.