St. Leo the Great Paschal Sermon II

For which reason the very feast which by us is named Pascha, among the Hebrews is called Phase, that is Pass-over, as the evangelist attests, saying, “Before the feast of Pascha, Jesus knowing that His hour was come that He should pass out of this world unto the Father.” But what was the nature in which He thus passed out unless it was ours, since the Father was in the Son and the Son in the Father inseparably? But because the Word and the Flesh is one Person, the Assumed is not separated from the Assuming nature, and the honour of being promoted is spoken of as accruing to Him that promotes, as the Apostle says in a passage we have already quoted, “Wherefore also God exalted Him and gave Him a name which is above every name.” Where the exaltation of His assumed Manhood is no doubt spoken of, so that He in Whose sufferings the Godhead remains indivisible is likewise coeternal in the glory of the Godhead. And to share in this unspeakable gift the Lord Himself was preparing a blessed “passing over” for His faithful ones, when on the very threshhold of His Passion he interceded not only for His Apostles and disciples but also for the whole Church, saying, “But not for these only I pray, but for those also who shall believe on Me through their word, that they all may be one, as Thou also, Father, art in Me, and I in Thee, that they also may be one in us.”

In this union they can have no share who deny that in the Son of God, Himself true God, man’s nature abides, assailing the health-giving mystery and shutting themselves out from the Easter festival. For, as they dissent from the Gospel and gainsay the creed, they cannot keep it with us, because although they dare to take to themselves the Christian name, yet they are repelled by every creature who has Christ for his Head: for you rightly exult and devoutly rejoice in this sacred season as those who, admitting no falsehood into the Truth, have no doubt about Christ’s Birth according to the flesh, His Passion and Death, and the Resurrection of His body: inasmuch as without any separation of the Godhead you acknowledge a Christ, Who was truly born of a Virgin’s womb, truly hung on the wood of the cross, truly laid in an earthly tomb, truly raised in glory, truly set on the right hand of the Father’s majesty.

— Pope St. Leo the Great, "On the Lord's Resurrection, II" (Sermon LXXII) .