Unhelpfully enough, this doesn't actually cover all Christians (such as Messianic Jews), and it most certainly doesn't do anything to inform us of who is actually practicing the religion and who simply says they are.
A slightly more complicated definition would be one who believes in the divinity of Jesus Christ and strives to live their life in accordance with His teachings.
Others keep the focus on the personal morality and sanctification, but express it through acts of charity and giving — many major charity organizations around the world are run by Christians, and many hospitals and ambulances worldwide originated as Christian organizations.
As you probably already know, Christians are not one collective bunch.
It is believed that the church's teachings on these subjects are "infallible" — without error — because the Holy Spirit will not allow the Church to be in error; debate remains, however on how to interpret this infallibility.
There are three sources of infallible teachings, two of which are not controversial.
While Christianity shares many of the same principles with Judaism and Islam, it is also very different because of influence from the Church Fathers and Roman culture: Most Christians can agree on at least that much. There are in-house disagreements on even these bare bones of its history.
Christianity spread throughout the Roman Empire, despite systematic persecution of Christians, and in the 4th century became the official religion of the Empire, and thereafter the dominant religion throughout Europe and the western world.The leader of the Catholic Church, The Pope, is the Bishop of Rome just as St.Peter was; in practice, the real authority of the Church is with its Bishops, each of whom is responsible for passing on the teachings of the Church intact within their dioceses.Like in most religions, disagreements over theology and dogma have resulted in everything from quiet splits to devastating wars, in the past and even today.In Christianity, this has resulted in the notable tendency to create new churches, and this in turn leads to the large number of different Christian churches.The world's biggest single religious group tends to be a bit misunderstood at times, even by its own adherents.