Two Christian contemporaries, one Latin, the other Greek, commented on the new situation resulting from the rise to sole power of a Christian emperor, an advent of which second-century Christians had dreamed. Tertullian (Apol. 21. 24) was sure emperors would be believers if they were not necessary for unspiritual duties in the secular world. After the establishment of Christianity by Theodosius I, Augustine could declare (En. in Ps. 93. 19) 'the emperor has become a Christian, but the devil has not'. Ambrose could write that every secular office is under Satan's power (in Luc. 4. 28). That kind of coolness towards a Christian empire was not characteristic of Lactantius or Eusebius, for whom it seemed almost the coming of the millennium, the fulfilment of Hebrew prophecy that God's word would spread throughout the inhabited world. It accompanied the realization that Christian missionaries of spiritual but no great intellectual power had enabled the gospel to take hold of the entire Roman empire with Persia, Armenia, Parthia, Scythia, and even Britain (Eus. Dem. Evang. 3. 5, p. 112d). Celsus had met Christians who looked forward to a Christian empire in which all the diversities of religion and morals would give way to a single law appropriate for the recognition of monotheism. Could empire and church together establish a global ethic? Eusebius (Dem. Evang. 8. 3, p. 407a) hailed Constantine's rise to sole power as happily ending all civil wars.
Diocletian's restructuring and reorganization of the empire reflected a sharp awareness of the huge problems of controlling a Mediterranean society, half of which spoke Greek, the other half Latin, besides tribal tongues which politically did not count. The civil wars among the members of his Tetrarchy will have enhanced this consciousness. Eusebius of Caesarea (Vita Const. 2. 19) observed that by his elimination of all rival emperors ('tyrants')