Chi-Ro

Diarmaid MacCulloch ('Rome's New Wave', LRB 2 June 2011) notes the Dark Age vogue for the Christian 'chi-ro' rebus:

This striking device, with no precedent in scripture or early Christian tradition, became an all-pervasive symbol of Imperial Christianity in the fourth century, appearing on the small change of imperial coinage ... [as well as] scratched on tiles, or as an ornament on silver dishes ...

The cross, particularly in its diagonalised format, has something of the headless Vitruvian about it: am emblem of humanity, God's and ours (Christ's incarnation and our redemption). But a headless body is not much of a body; and the ro adds not only a head but a splendidly exaggerated male member too: the sword of the lord, the phallus of grace. No wonder it caught on.