The peak was reached and passed early in the third century A.D., when Jerash was promoted to the rank of colony, and the grade is steadily downhill after that, with an occasional level stretch or even a little rise; but the best was over. But it was a gradual descent closely connected with the fortunes of the Roman Empire, and for Jerash there were no precipices on the road. No more buildings were erected in the grand style, and already by the end of the century we find carved and even inscribed blocks being carelessly re- used in building, always a bad sign. The destruction of Palmyra and the growth of the Sassanian Kingdom in Iraq effectively put a stop to big- scale commerce and shifted trade routes away from the east. Cities like Jerash, almost on the eastern border, must have felt the effect at once, and with the weakening of Roman force the old predatory instincts of the Arab tribes came to the surface again and security became doubtful. But under Diocletian the Sassanians were defeated and there was a short level stretch during which some building, such as the circular plaza and the shops around the South Tetrapylon, was carried out. The work, however, was slipshod, though not quite so bad as later Byzantine building, and many of the inscriptions of the period are cut on earlier pedestals or columns or even on top of partly defaced earlier inscriptions.

By the middle of the fourth century there was a large Christian community in Jerash, and the Cathedral and the Fountain Court were flourishing, for the writer Epiphanius states that some of his contemporaries had drunk from the fountain at Gerasa, whose waters turned to wine each year at the anniversary of the miracle of Cana. But from the town itself there is little history to be gleaned in the fourth century; inscriptions are conspicuous by their absence, and the only other outside reference tells that the Christians were represented at the council of Selecucia in 359 by the Bishop Exeresius. Bishop Plancus represented them at the council of Chalcedon in 451, by which time Christianity must have become the ruling religion of the town. In 440- 442 some repairs to the fortifications were carried out; the Church of the Prophets, Apostles and Martyrs was built in 464- 5, and that of St. Theodore in 464- 6, when the fountain court was also remodelled.

Under Justinian, 531- 565, there was a rise in prosperity, and no fewer than seven churches are known to have been erected in this period. Inscriptions record the erection of other public buildings of an unidentifiable nature, and even the revival of the pagan Maiumas water festival in 535. Many of the churches have been excavated, and from the objects found in them and in related buildings we can get a good idea of the life of the time. Low though the standard might be in comparison with former splendours, there was none the less a fair degree of rather cheap luxury. Appearances were all that mattered and beauty was only just skin deep. Gleaming marble and brightly coloured glass mosaics on the walls of the churches concealed a type of construction than which it would be hard to imagine worse. As the main centres of life in this period were around the churches, it naturally reflected their style. The gaily dressed women who crowded the shops and drifted in and out of the churches were adorned with magnificent strings of beads of precious stones and gold ear- rings and ornaments, which on close inspection turned out to be glass imitations and thinly gilded bronze. Still, it was all very pretty on the surface, and life was by no means unpleasant or difficult. There were new baths built by the Bishop Placcus next door to St. Theodore’s Church for the use of parishioners, perhaps the earliest example of “cleanliness being next to godliness.” The choristers had a club room just across the road from the church, and the clergy were provided with extensive and comfortable quarters adjoining the forecourt.

All this external beauty and comfort was only achieved at the cost of the earlier buildings, particularly temples. An orgy of destruction of the pagan shrines must have gone on, and it seems as though scarcely one new stone was cut for the construction of any of the churches. The beautiful courtyard of the Artemis Temple was desecrated by the building of potters’ kilns there.

The last church of which- we know at present is that built by Bishop Genesius in 611, and the Persian invasion of 614 was the beginning of the end of Jerash. The only remains of this invasion are goal- posts erected in the Hippodrome just outside the South Gate for playing polo. The Muslim conquest in about 635 completed the decline of the city, which, though it continued to be occupied, gradually shrank to about a quarter of its original size.