"Going back to closed societies is neither possible nor desirable, nor is it a Christian option," Marx said.
In Cologne Protestant, Catholic, Orthodox and Free Churches openly spoke out against the far-right AfD (Alternative for Germany), which held its party conference in the city on the Rhine in late April.
The churches called for a clear commitment to democracy.
Christians had taken to the streets to demonstrate for "openness to the world, tolerance and solidarity," said the Archbishop of Cologne, Cardinal Rainer Maria Woelki.
Their motto was 'Our Cross is not hooked' - to distinguish it from the Swastika.