III. The acknowledgment of our nature in Christ is necessary to orthodoxy.
As these iniquitous lies were once rejected by the Catholic Faith, and such men's blasphemies condemned by the unanimous votes of the blessed Fathers throughout the world, whoever these are that are so blinded and strange to the light of truth as to deny the presence of human, that is our, nature in the Word of God from the time of the Incarnation, they must show on what ground they claim the name of Christian, and in what way they harmonize with the true Gospel, if the child-bearing of the blessed Virgin produced either the flesh without the Godhead or the Godhead without the flesh.
For as it cannot be denied that “the Word became flesh and dwelt in us,” so it cannot be denied that “God was in Christ, reconciling the world to Himself.” But what reconciliation can there be, whereby God might be propitiated for the human race, unless the mediator between God and man took up the cause of all? And in what way could He properly fulfil His mediation, unless He who in the form of God was equal to the Father, were a sharer of our nature also in the form of a slave: so that the one new Man might effect a renewal of the old: and the bond of death fastened on us by one man's wrong-doing might be loosened by the death of the one Man who alone owed nothing to death.
For the pouring out of the blood of the righteous on behalf of the unrighteous was so powerful in its effect, so rich a ransom that, if the whole body of us prisoners only believed in their Redeemer, not one would be held in the tyrant's bonds: since as the Apostle says, “where sin abounded, grace also did much more abound.” And since we, who were born under the imputation of sin, have received the power of a new birth unto righteousness, the gift of liberty has become stronger than the debt of slavery.
Source: Letters (New Advent)