From the “Devil’s Final Battle”:
And then, on July 13, 1917, the Lady had shown them something which would terrify them and change them forever, making them into saints who would spend their lives (in the case of Francisco and Jacinta, very brief lives) praying and making sacrifices for sinners. As Lucy recounts in testimony the Catholic Church has deemed worthy of belief, the Lady showed them hell:
She opened Her hands once more, as She had done during the two previous months. The rays of light seemed to penetrate the earth, and we saw as it were a sea of fire. Plunged in this fire were demons and souls [of the damned] in human form, like transparent burning embers, all blackened or burnished bronze, floating about in the conflagration, now raised into the air by the flames that issued from within themselves together with great clouds of smoke, now falling back on every side like sparks in huge fires, without weight or equilibrium, amid shrieks and groans of pain and despair, which horrified us and made us tremble with fear. (It must have been this sight which caused me to cry out, as people say they heard me). The demons could be distinguished [from the souls of the damned] by their terrifying and repellent likeness to frightful and unknown animals, black and transparent like burning coals.2 This vision lasted but an instant. How can we ever be grateful enough to our kind heavenly Mother, Who had already prepared us by promising, in the first apparition, to take us to Heaven. Otherwise, I think we would have died of fear and terror.3
Illuminating book.