Origin of gospel truth
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.