Why Old Believers are forbidden to shave a beard

Rusichi since ancient times carried beards, as before adoption of Christianity as official religion the prince Vladimir in 988, and after that epoch-making event. Practically each adult Russian man wore traditional clothes and a beard as well in the Middle Ages, and during earlier modern times. And only Peter I, with his program which present words could be characterized as "we will catch up and we will overtake Europe", intended to deprive of the Russian men of a beard. For what it was necessary for it? The matter is that, having visited Europe, Pyotr saw that local gentlemen wear wigs and shave the person...