Such was the state of all mortals resulting from our first ancestors that…no one would have escaped the punishment of condemnation, had not the Word become flesh and dwelt in us, that is to say, in that nature which belonged to our blood and race.
And accordingly, the Apostle says: “As by one man’s sin (judgment passed) upon all to condemnation, so also by one man’s righteousness (it) passed upon all to justification of life.
For as by one man’s disobedience many were made sinners, so also by one man’s obedience shall many be made righteous” (Rom. 5:18, 19);
and again, “For because by man (came) death, by man also (came) the resurrection of the dead.
And as in Adam all die, so also in Christ shall all be made alive” (1 Cor. 15:21, 22).
All they to wit who though they be born in Adam, yet are found reborn in Christ, having a sure testimony both to their justification by grace, and to Christ’s sharing in their nature;
for he who does not believe that God’s only-begotten Son did assume our nature in the womb of the Virgin-daughter of David, is without share in the Mystery of the Christian religion, and, as he neither recognizes the Bridegroom nor knows the Bride, can have no place at the wedding-banquet.
For the flesh of Christ is the veil of the Word, wherewith every one is clothed who confesses Him unreservedly.
[...] Hence whosoever confesses not the human body in Christ, must know that he is unworthy of the mystery of the Incarnation, and has no share in that sacred union of which the Apostle speaks, saying, “For we are His members, of His flesh and of His bones.
For this cause a man shall leave father and mother and shall cleave to his wife, and there shall be two in one flesh” (Eph. 5:30-32). And explaining what was meant by this, he added, “This mystery is great, but I speak in respect of Christ and the Church.”
Therefore, from the very commencement of the human race, Christ is announced to all men as coming in the flesh.
In which, as was said, “there shall be two in one flesh,” there are undoubtedly two, God and man, Christ and the Church, which issued from the Bridegroom’s flesh, when it received the mystery of redemption and regeneration, water and blood flowing from the side of the Crucified.
For the very condition of a new creature which at baptism puts off not the covering of true flesh but the taint of the old condemnation, is this, that a man is made the body of Christ, because Christ also is the body of a man.
Leo the Great (c.400-461): Letter 59, 4.