Today and tomorrow, 27 & 28 October, are key dates in Christian history. Constantine's 'vision of the Cross' in 312, and his attribution of military victory at the Battle of Milvian Bridge the next day to God, was the beginning of Christendom in Europe - an era which mixed civilisation with bloodshed, saints with militarism, and faith with often brutal sacralised-secular power.
With the defeat of Maxentius, Constantine became the sole Roman Emperor. The subsequent decree of religious toleration, which established Christianity alongside others in the Imperial pantheon, spared Christians persecution - but embroiled them in power politics in a way which bypassed or reversed key aspects of the Gospel message. The Jesus who refused violence and favoured the poor, women and the despised became an embarassment. He was replaced with an imperial version of Christ, degrading both his humanity and divinity.
The disease involved in all this was widespread. On 27 October in 1553 Michael Servetus burned as a heretic just outside Geneva. On the same day in 1659, two Quakers who came to America from England were executed in Massachusetts Bay Colony for their beliefs.
Yet embedded in this disturbing history (not least in its victims) there is another, liberating story. The Anabaptists, the Quakers and other non-conformists refused state religion and violence. In the nineteenth and twentieth century Christians campaigned for social justice and helped form peace movements which had a worldwide impact.
On 27 October 1967 Catholic priest, theologian and activist Philip Berrigan and three others (the 'Baltimore Four') protestested against the Vietnam War by pouring their own blood on Selective Service records - a very different understanding of the meaning of the Cross to the one Constantine perpetuated.
Then on 27 October 1968, 120,000 marched against war in London, urged on by campaigning Anglican priest Canon John Collins, one of a range of clerical heroes of the anti-apartheid struggle.
Here we see the two sides of Christianity: not so much 'liberal' versus 'conservative', but an imperial version of the faith captive to earthly power, contrasting with a non- or post-imperial (and now post-Christendom) understanding of the Gospel, recovering the dynamic of the originating Jesus movement.
This evening, my Ekklesia colleague Symon Hill, author of 'The No-Nonsense Guide to Religion' (http://www.newint.org/books/no-nonsense-guides/religion/ ), published recently by New Internationalist Publications, has been talking about war religion and peace religion at Housmans Bookshop in Central London.
Symon chose a significant day to engage in conversation with both allies and sceptics over all this, and his book charts the divergent strands in the historic religions - the understanding of which remains so vital to the quest to overcome violence and dominating power in the world today.
(c) Simon Barrow is co-director of Ekklesia. (He worked above Housmans Bookshop in 1976, as a volunteer for the Campaign Against Arms Trade, whose steering committee he served on from 1977-1987.)