The Christianity of the Egyptians was mostly superstition; and as it spread over the land it embraced the whole nation within its pale, not so much by purifying the pagan opinions as by lowering itself to their level, and fitting itself to their corporeal notions of the Creator.  This was in a large measure induced by the custom of using the old temples for Christian churches; the form of worship was in part guided by the form of the building, and even the old traditions were engrafted on the new religion.  Thus the traveller Antonius, after visiting the remarkable places in the Holy Land, came to Egypt to search for the chariots of the Egyptians who pursued Moses, petrified into rocks at the bottom of the Red Sea, and for the footsteps left in the sands by the infant Jesus while he dwelt in Egypt with his parents.  At Memphis he enquired why one of the doors in the great temple of Phtah, then used as a church, was always closed, and he was told that it had been rudely shut against the infant Jesus five hundred years before, and mortal strength had never since been able to open it.

The records of the empire declared that the first Caesars had kept six hundred and forty-five thousand men under arms to guard Italy, Africa, Spain, and Egypt, a number perhaps much larger than the truth; but Justinian could with difficulty maintain one hundred and fifty thousand ill-disciplined troops, a force far from large enough to hold even those provinces that remained to him.  During the latter half of his reign the eastern frontier of this falling empire was sorely harassed by the Persians under their king Chosroes.  They overran Syria, defeated the army of the empire in a pitched battle, and then took Antioch.  By these defeats the military roads were stopped; Egypt was cut off from the rest of the empire and could be reached from the capital only by sea.  Hence the emperor was driven to a change in his religious policy.  He gave over the persecution of the Jacobite opinions, and even went so far in one of his decrees as to call the body of Jesus incorruptible, as he thought that these were the only means of keeping the allegiance of his subjects or the friendship of his Arab neighbours, all of whom, as far as they were Christians, held the Jacobite view of the Nicene creed, and denied the two natures of Christ.

As the forces of Constantinople were driven back by the victorious armies of the Persians, the emperors had lost, among other fortresses, the capital of Arabia Nabataae, that curious rocky fastness that well deserved the name of Petra, and which had been garrisoned by Romans from the reign of Trajan till that of Valens.  On this loss it became necessary to fortify a new frontier post on the Egyptian side of the Elanitic Gulf.  Justinian then built the fortified monastery near Mount Sinai, to guard the only pass by which Egypt could be entered without the help of a fleet; and when it was found to be commanded by one of the higher points of the mountain he beheaded the engineer who built it, and remedied the fault, as far as it could be done, by a small fortress on the higher ground.  This monastery was held by the Egyptians, and maintained out of the Egyptian taxes.  When the Egyptians were formerly masters of their own country, before the Persian and Greek conquests, they were governed by a race of priests, and the temples were their only fortresses.