Council of Chalcedon, Canon VII

We have decreed that those who have once been enrolled among the clergy, or have been made monks, shall accept neither a military charge nor any secular dignity; and if they shall presume to do so and not repent in such wise as to turn again to that which they had first chosen for the love of God, they shall be anathematized.

Translation: Henry R. Percival, The Seven Ecumenical Councils, NPNF II.14 (1900), via CCEL.