‘Gospel’ is not exclusively Christian in origin; the birthday of the Emperor Augustus was described, in a pagan inscription, as ‘good tidings’ for the world. ‘Gospel’ has more than one meaning in Christian usage: generally, it refers to the content of the Christian revelation as a whole, the good news of salvation and the kingdom of God; more particularly it can refer to the content of Jesus’s own preaching.
In the second century AD the word came to be used as the title of each of the four New Testament books setting forth the Christian Gospel. Strictly speaking, there is only one Gospel, not four, which is why these books are often headed with the form of words “The Gospel according to …”. The authors of these four books are often called the ‘evangelists’, for they are the narrators of the euaggelion, the good news. There are other ‘Gospels’ outside the canonical ones, such as the Gospel of Thomas, which are of interest to the religious historian.