Originally I intended this to be a much more informative and in-depth sort of post, but I had a lot of trouble finding sources. In my FSEM (When Women Ruled Russia), we had read a part of Alexander Radishchev's A Journey From St. Petersburg to Moscow. One of the things that spawned discussion was when Radishchev saw a peasant working on a Sunday and said, "You are, of course, a Raskolnik [religious dissident], since you work on Sunday." The peasant replied, "No, sir, I make the correct sign of the cross," and showed his three fingers put together.
So then Dr. Steeves went on a ramble (an interesting and informative ramble, but a ramble nonetheless) about the history of how one crossed oneself.
Here's where the problems come in: I couldn't really find any internet sources on the history of the subject. All I really have to go off of is Dr. Steeves' long-winded explanation. But I'll tell you what I remember.
Around the time of Ivan the Terrible (1530 - 1584), it was established that the correct way to cross oneself was to use two fingers. However, in 1666 Patriarch Nikon of the Russian Orthodox Church decided to examine the Greek Orthodox Church (since the ROC originated from the Greek Orthodox Church), and saw that the Greeks made the sign of the cross using three fingers. So he took that back to Russia and declared that using three fingers was the "right way". But the Old Believers thought, "Nooo, we've always been taught two fingers. That's the right way." and this disagreement over liturgical reform is one of the factors that caused a schism in the church. Well, the Russians believed that when the church started to fall apart, it was a sign of the coming of the Antichrist, and since the Patriarch had brought about this change, he must be the Antichrist! Moreover, the church authorities met in 1666, and 666 is The Mark of the Beast. Which, of course, only served to reinforce the Old Believers in their conviction that two fingers was the true way to do it.
The symbolism behind the number of fingers is that the three fingers is supposed to represent the Trinity. But Old Believers argued that crossing oneself was not about God, rather about Jesus, and one should therefore use two fingers; one to represent the human part of Jesus, the other to represent the divine.
Don't quote me on any of this. I still need to fact check and whatnot. I just thought it was an interesting topic, because of how worked up everyone got over such a seemingly minuscule detail.