During the first 400 years of Christianity's presence in the world, it changed dramatically. It began modestly in an area near the eastern Mediterranean, but over the course of 400 years it has spread to the entire Mediterranean and surrounding areas. Throughout this period of growth, there were also many changes in the beliefs of Christianity, with the main belief centered on Jesus Christ. As Christianity grew and entered new areas, it was introduced to new people who interpreted the different scriptures and preached what it meant to be Christian. Some of the most influential writings of these years came from Irenaeus of Lyons, a 2nd-century writer, Christian, theologian, and bishop. He expressed in his writings his beliefs about what made a Christian a Christian. One of his most famous writings, The Demonstration of Apostolic Preaching, highlights the idea of ​​the Holy Trinity and the Rule of Faith in the interpretation of the Bible. Irenaeus believed that the Rule of Faith was ultimately necessary and required when reading and interpreting the word of Christianity saying, in the words of Isaiah: “Except you believe, neither will you understand” ( paragraph 3). In Irenaeus's eyes, the Rule of Faith is one of the main things that makes a Christian a Christian. The first part of the rule of faith reminds Christians “God, the Father, not created, not material, invisible; one God, creator of all things” (paragraph 5). Irenaeus believes this is important when interpreting the Bible, because God the Father is the creator of all things and "by his Word he created the whole world" and "gave laws that everyone must obey" ( paragraph 10). Irenaeus says that since God is in the middle of the paper, it comes from Him” (paragraph 100). As stated previously, Irenaeus places a lot of emphasis on belief in the three characters. Ultimately, Irenaeus believes that the rule of faith is one of the major attributes that Christians must possess to be Christians. He supports this by mentioning the reasons why the Rule of Faith is so essential for true Christians to remember. He goes further by giving arguments against his point of view and omits them by giving his truth about what the Rule of Faith is. Overall, while many major beliefs may come from Christianity, the major belief that aids in the interpretation of Scripture and the Bible can be seen in the Rule of Faith. The rule of faith is meant to serve as a guide and reminder of the truths of Christianity. Without the rule of Faith, it would be easy to get lost and forget what the truth really is..