Moscow is now home to 2 million Muslims - more than any other city in Europe. So when the faithful gathered Tuesday for the start of the Eid al-Adha holiday, tens of thousands of men unrolled their prayer mats on the asphalt of Moscow’s streets. Last year, they prayed outside in rain and snow. Looming overhead were the new minarets of the Moscow Cathedral Mosque, a century-old mosque that is now being expanded. It is one of only four mosques in all of Moscow. “Certainly mosques are needed. Mosques are needed in each micro-district,” said Abdul Bari Sultanov, a Russian...