the gospel truth

The absolute or indisputable truth. I was home all night, and that's the gospel truth—Mom can confirm it.

Fig. the undeniable truth. The witness swore he was telling the gospel truth. I told my parents the gospel truth about how the vase broke.

Something that is unquestionably true. For example, Every word he uttered was the gospel truth. The word gospel, which comes from the Old English god spel, "good news," has been used to describe something that is thought to be as true as the biblical gospel (that is, undeniably true) since the 13th century. The current idiom originated in the 1600s, when it referred to biblical truths, and has been applied to truth of a more general nature since the late 1800s. Also see take as gospel.

Something that may safely be believed. This term dates from the Middle Ages, when Christianity was almost universally accepted in Western civilization. Thus Chaucer used “gospel” in numerous places to mean incontrovertible truth. The word “gospel” is actually a corruption of the Old English godspel, meaning “good tidings,” and was used to signify the glad tidings preached by Jesus, the life of Jesus as told in the New Testament (whose first four books are generally referred to as the Gospels), and the religious doctrine set forth there. Thus gospel truth literally means something as true as what is contained in the Gospels, which once were believed to be literally true, and the term has survived universal belief in that faith by a good many years.