Christianity is important because it has a prominent role in history, and is one of the world's leading religions. Christianity has the largest numbers of adherents of any religion, with more than two billion people following its teachings. Those teachings are important to those who believe, because among them are the keys to having a deeper relationship with God, as well as living a just and moral life, which will allow them to reach Heaven after death.
Christianity's role in the world has changed at many points throughout history, and there have been times (such as in the second century, and even in some modern day countries) when Christians have been persecuted for their beliefs. On the other hand, there have been many points when this religion has been dominant in the world, or its followers have been intolerant of other non-Christian religions.
In terms of religious importance, Christians have many teachings about fellowship, how to have a strong relationship with God, and how to live a good life. The Christian religion is extremely important to its believers, as it directly affects how many of them go about their daily lives, and how they treat others. Christianity shares the roots of its faith with Judaism and Islam.