Almost certainly Russian Orthodox. The Communists associated the Church with the old regime and tried to obliterate it. They nearly succeeded, at least in numbers. We will never know the exact number of martyrs but it is likely that upwards of ten million Orthodox Christians were liquidated by the Communists. More than half under Stalin.
Today the Orthodox Church commemorates the Holy New Martyrs of Russia annually on January 25 (o.s.) / February 7 (n.s.).
Holy New Martyrs of Russia Pray for Us!