The English word means ‘news of joyful events’ and is used by Jesus as he proclaims the coming of the Kingdom (Mark 1: 15) and by Paul of God's work done through Jesus Christ (Rom. 1: 1–2). The NT takes up the theme already preached by the OT prophet, where it is the good news of the coming end of the Exile (Isa. 40: 9). In writing to the Galatians (1: 6–7) Paul rounds on opponents who were distorting ‘his’ gospel and teaching ‘another’ which he regarded as nothing more than the old way of Judaism slightly improved or modified. It was the old legalism. ‘His’ gospel was the news that it is by Christ, through faith in him, that we are put right with God. Thus the content has been changed: for Jesus, it was the coming of the Kingdom: for Paul, it is Christ himself ‘the power of God for salvation’ (Rom. 1: 16). The content of the gospel has necessarily been changed by reason of the Church's belief about Jesus' death and resurrection.
In the 2nd cent. ‘gospel’ came to be put as the title of certain Christian books. This followed the convention that a book should be known by its first word: thus, ‘Genesis’, the opening book of the OT, acquired its name from the LXX where the word means ‘beginning’ (Gen. 1: 1). Almost the first word in Mark is ‘gospel’; but before long there were three other accepted accounts of Jesus' ministry, death, and resurrection; therefore to distinguish them their titles became ‘The gospel according to Matthew’ etc. The word ‘gospel’ had come to denote a kind of literature about Jesus, but it was not an entirely new genre of writing inasmuch as lives of political or military leaders (Agricola) and religious heroes (Moses) were published in the Graeco-Roman world.