Answer
Islam was the primary religion of the Volga Bulgars during Ibn Fadlan's visit in the 10th century. The Volga Bulgars were a Turkic people who established a powerful state in the Volga-Ural region of Eastern Europe. They had converted to Islam in the 9th century, and by the time of Ibn Fadlan's visit, Islam was firmly established as the dominant religion among the Volga Bulgars.