To see just how mainstream these alternative texts were in the medieval churches, consider one image that pervades the religious art of the era, namely the Harrowing of Hell. Drawn from the popular Gospel of Nicodemus, the Harrowing was not far behind the Crucifixion as a theme in medieval Christian imagery, and its impact extended to art, drama and literature. An image fundamental to mainstream Christian belief for over a thousand years derived entirely from an ancient alternative gospel.The passages in 1 Peter 3:18-21, 4:6 (cf. 2 Peter 2:4, Jude 6) partly inspire the Harrowing of Hell myth (a.k.a. the Descensus ad Infernos). They in turn arguably have connections with the myth of the fall and infernal banishment of the Watchers in 1 Enoch
We see from Jude 14-16 that first-century Jesus followers were reading 1 Enoch. Some even considered it prophecy. For more on that, see here and here.
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