197 - Clodius Albinus, governor of Britain, another claimant to the Imperial throne, is killed by Severus at the battle of Lyon
208 - Severus goes to defend Britain, and repairs Hadrian's Wall
209 - St. Alban, first British martyr, was killed for his faith in one of the few persecutions of Christians ever to take place on the island, during the governorship of Gaius Junius Faustinus Postumianus (there is controversy about the date of Alban's martyrdom. Some believe it occurred during the persecutions of Diocletian, in the next century, although we opt for the earlier dating).
c.270 - Beginning (highly uncertain dating) of the "Saxon Shore" fort system, a chain of coastal forts in the south and east of Britain, listed in a document known as "Notitia Dignitatum."
287 - Revolt by Carausius, commander of the Roman British fleet, who rules Britain as emperor until murdered by Allectus, a fellow rebel, in 293
303 - Diocletian orders a general persecution of the Christians
306 - Constantine (later to be known as "the Great") was proclaimed Emperor at York.
311 - Persecution of Christians in the Roman Empire ends.
312 - Constantine defeats and kills Maxentius at battle of Milvian Bridge; Constantine realizes Christian God may be a powerful ally and decides to attempt to co-opt him for his own purposes.
313 - Edict of Toleration proclaimed at Milan, in which Christianity is made legal throughout the empire.
314 - Three British bishops, for the first time, attend a continental church gathering, the Council of Arles.
324 - Constantine finally achieves full control over an undivided empire. He was a skillful politician who is popularly believed to have made Christianity the official religion of the empire because of his personal convictions. In actuality, that act was merely an expedient intended to harness the power of its "God" for the benefit of the state. He re-located the imperial headquarters to Byzantium, whose name he then changed to Constantinople.
Despite his outward enthusiasm for Christianity and its powerful God, he didn't close many pagan temples during his reign. He did, however, strip them of their former wealth, which was then shifted to various Christian churches. This produced the result that many of the fledgling churches were put on a very firm financial footing and many of their members enjoyed great prosperity. The persecution of Christianity had stopped, perhaps, but its co-opting had just begun.
Early Christianity had no official hierarchies and functioned best as a series of small church groups worshipping with and caring for their own members while spreading the Gospel in their local areas. Constantine's move created a top-heavy structure that would quickly depart from its original purity; a church beholden to the state, out of touch with the needs of its adherents and concerned only with its own comfort. Eusebius, the early Christian historian, has given us some additional insights into the motivations of the Emperor Constantine in his "Ecclesiastical History"
337 - Constantine received "Christian" baptism on his deathbed. Joint rule of Constantine's three sons: Constantine II (to 340); Constans (to 350); Constantius (to 361)