They who are of the priesthood, or of the clergy, shall not be magicians, enchanters, mathematicians, or astrologers; nor shall they make what are called amulets, which are chains for their own souls. And those who wear such, we command to be cast out of the Church.
Ancient Epitome of Canon XXXVI.
Whoso will be priest must not be a magician, nor one who uses incantations, or mathematical or astrological charms, nor a putter on of amulets.
Some interesting and valuable information on charms will be found in Ducange (Glossarium, s.v. Phylacterea).
“Magicians” are those who for any purpose call Satan to their aid. “Enchantors” are those who sing charms or incantations, and through them draw demons to obey them. “Mathematicians” are they who hold the opinion that the celestial bodies rule the universe, and that all earthly things are ruled by their influence. “Astrologers” are they who divine by the stars through the agency of demons, and place their faith in them.
Zonaras also notes that the science of mathematics or astronomy is not at all hereby forbidden to the clergy, but the excess and abuse of that science, which even more easily may happen in the case of clergymen and consecrated persons than in that of laymen.