As of the early 21st century, it has between 1.5 billion and 2.1 billion adherents, representing about a quarter to a third of the world's population.
There are several different forms of Christianity; all of them go back to the early Christian churches founded in the first century A.D. The biggest Christian denominations that still exist are Catholicism (which includes both Roman-Catholicism and Anglicanism), Protestantism (Which can be divided into Lutheranism, Calvinism and Evangelism and several forms of Eastern Christianity spread in Eastern-Europe and the Middle-East.
Jesus is the only lord. Son of God. Performed miracles. Came to heal us. Died for our sins.