Final answer:
Patriarch Nikon's reforms initiated the Old Believer controversy, leading to a significant schism within Russian Orthodoxy over liturgical practices.
Step-by-step explanation:
The reforms that began the Old Believer controversy were introduced by Nikon, who was the Patriarch of Moscow from 1652 to 1666. His attempts to standardize Russian liturgical practices in line with those of the Greek Orthodox Church led to a schism within Russian Orthodoxy. Those who rejected Nikon's reforms became known as the Old Believers. The controversy centered on rituals such as the correct way to make the sign of the cross, the pronunciation of Jesus' name, and the number of loaves used in the Eucharist. This controversy had long-standing implications for the Russian Orthodox Church and continues to have significance in the understanding of Russian religious history.