MacIan is arrested shortly after his arrival in London... why?

When Macian arrives in the city, he finds a shop called The Atheist, which is a publication run by Mr. James Turnbull. MacIan stops to read an article posted in the front window of the shop, and when he discovers Turnbull has blasphemed the Virgin Mary in his writing, he shatters the shop window with a rock. He is quickly arrested.