Miniature of Leo I, contained in the Menologion of Basil II (c. 1000 A.D., Vatican Library). Via Wikipedia
O Lord our God, grant that your Church, following the teaching of your servant Leo of Rome, may hold fast the great mystery of our redemption, and adore the one Christ, true God and true Man, neither divided from our human nature nor separate from your divine Being; through the same Jesus Christ our Lord, who lives and reigns with you and the Holy Spirit, one God, now and for ever. Amen.
— Book of Common Prayer