'three fingers...or the traditional two'

Devout Russians considered crucial such matters as how many hallelujahs were to be shouted at various points in the service, how many consecrated loaves were to be at the offertory or on the altar, the spelling of Jesus' name (from Isus to Iisus) and, most notably, whether, in making the sign of the cross, one extended the newly decreed three fingers (symbolizing the Trinity) or the traditional two fingers (symbolizing the dual nature of Christ). If one was convinced that the world was only a preparation for paradise or the inferno, and that personal salvation depended on the punctilious observance of church ritual, then crossing oneself with two fingers instead of three could make the difference between spending eternity in heaven or in hellfire.

—Robert K. Massie, Peter the Great: His Life and World (1980)