“The New Testament is a book in which are written the gospel and the promises of God, together with the history of those who believe and of those who do not believe them.
For ‘gospel’ [Euangelium] is a Greek word and means in Greek a good message, good tidings, good news, a good report, which one sings and tells with gladness.
For example, when David overcame the great Goliath, there came among the Jewish people the good report and encouraging news that their terrible enemy had been struck down and that they had been rescued and given joy and peace; and they sang and danced and were glad for it [I Sam. 18:6].
Thus this gospel of God or New Testament is a good story and report, sounded forth into all the world by the apostles, telling of a true David who strove with sin, death, and the devil, and overcame them, and thereby rescued all those who were captive in sin, afflicted with death, and overpowered by the devil.
Without any merit of their own He made them righteous, gave them life, and saved them, so that they were given peace and brought back to God. For this they sing, and thank and praise God, and are glad forever, if only they believe firmly and remain steadfast in faith.
This report and encouraging tidings, or evangelical and divine news, is also called a New Testament. For it is a testament when a dying man bequeaths his property, after his death, to his legally defined heirs.
And Christ, before His death, commanded and ordained that His gospel be preached after His death in all the world [Luke 24:44–47]. Thereby He gave to all who believe, as their possession, everything that He had.
This included: His life, in which He swallowed up death; His righteousness, by which He blotted out sin; and His salvation, with which He overcame everlasting damnation.
A poor man, dead in sin and consigned to hell, can hear nothing more comforting than this precious and tender message about Christ. From the bottom of his heart he must laugh and be glad over it, if he believes it true.”
–Martin Luther, “Preface to the New Testament (1522)” in Luther’s Works, Vol. 35: Word and Sacrament I, ed. Jaroslav Jan Pelikan, Hilton C. Oswald, and Helmut T. Lehmann, vol. 35 (Philadelphia: Fortress Press, 1999), 358–359.