The Emperor Constantine, by Michael Grant (Phoenix Giant, £10.99 in UK)

It remains a subject of debate whether or not Constantine the Great ever was a Christian in any orthodox sense, but it seems …

It remains a subject of debate whether or not Constantine the Great ever was a Christian in any orthodox sense, but it seems unarguable that it was he who established Christianity - previously a suspect sect, and at times a persecuted one - as the official religion of the Roman Empire. His vision of a cross appearing in the sky, before his battle with his rival Maxentius at the Milvian Bridge at Rome, appears to have been genuine, although later he used it adroitly for propaganda reasons. Constantine was a man of great ability and also of great ruthlessness, both qualities which helped to stabilise the ageing, weakening empire for a time after decades of upheaval and civil war. Michael Grant, however, finds his personality enigmatic and somehow fails to bring him into a really clear focus. Jacob Burkhardt's once-classic study of 4th-century Rome and its many ephemeral rulers should be republished; its horizons are wider and its insights deeper than this useful but rather pedestrian biography.

Brian Fallon