We all know how fast and far a good tune can travel. Shortly after the haunting melody of “Lili Marleen” was heard broadcast to among German troops in WWII, it was eagerly adopted by Allied ones. And think, too, of the travels of a song which began as “God preserve Franz the Kaiser”. This lovely tune by Haydn was recycled as “Deutschland, Deutschland über alles” and eventually appeared as the hymn, “God who touches earth with beauty”.
Just as a catchy tune travels, so does a powerful image. The ornate leaf masks or foliated heads, now often called the “Green Man”, were developed by Roman artists in connection with nature gods like Dionysus and the satyrs. But this image was too good to end with Roman paganism. In the Middle Ages, as the pageants of chivalry became widespread, Green Men appear with increasing frequency in Christian churches, too.
A clue to this is offered by a ritual from 16th-century Sweden. There we find a May Day jousting contest between Winter and Summer. The idea was to dramatise — and help advance — the coming of spring. The two sides in the tournament were clothed accordingly. Duke Winter was “clad in various pelts and armed with pokers, scattering ice and snowballs to prolong the cold”. His opponent, Count Floral, was “garbed in the green boughs of trees, together with leaves and flowers”. Of course, whatever the weather on the day of the tournament, “and to everyone’s joy, the victory is awarded to Summer”. 
This gives us a glimpse into the mediaeval meaning of faces decked with foliage. No longer were they Roman gods, but harbingers of spring. And, of course, spring in the metaphorical sense, was a central theme in Christianity. It applied to both the rebirth of the soul in heaven and the general Resurrection of the Dead. As such, the foliated head had a firm claim to its place in Christian churches.
But the transformations didn’t end there. The Green Man could represent, not only the resurrection of the soul, but also the rebirth of the Christianity during the Reformation. This seems to be why the Reformers adopted it initially. (However, they soon backed off from this transformed satyr. After all, they were the ones who liked to accuse the Catholic Church of being “pagan”.) But at first, presumably as a symbol of the rebirth of the church, a Green Man appears in a portrait of Martin Luther by Cranach the Elder, also on the title page of Luther’s petition to the Papal Council in 1520, and even in his church in Wittenburg. 
Mediaeval people loved symbols precisely because they regarded them as having magic power. This symbol is simple, vivid, and closely connected to people’s hopes and fears: to their longing for the return of the sun — to a “springtime” through resurrection — and, for some, to a new and reformed Christianity. It’s no wonder that the Green Man, in its various guises, survived for two millenia.
1. Olaus Magnus, Description of the Northern Peoples (1555), Volume 2, 15:9.
2. Clive Hicks, http://www.geomantie.net/article/read/6093.html