Roman Catholicism affirms the dogmatic teachings of Chalcedon from AD 451, including the Hypostatic Union, wherein Jesus, while having two natures (being fully God and fully man), is a single person. However, other dogmas affirmed by Rome (viz. Transubstantiation and the Immaculate Conception) undermines the humanity of Jesus within the sphere of Trinitarian Christology.
Commenting on the theological implications of Transubstantiation, as defined at the Fourth Lateran Council in 1215, Kenneth Collins and Jeffrey Walls notes the following:
First of all, and bearing in mind the original setting of the Lord’s Supper, if the elements are substantively the body and blood of Christ, then this can only mean, among other things, that Christ had two bodies. By this understanding, at the original Lord’s Supper Christ, with full bodily integrity, was holding the bread, which itself was fully another body of Christ (“Christ is present the whole and entire in each of the species”). However, the formula of the ancient church, evident at Chalcedon, was one person with two natures, which means that Christ as the God/Human had only one body (even the Monophysites could agree on this last point): “We . . . confess one and the same Son, our Lord Jesus Christ, the same perfect in Godhead and also perfect in manhood; truly God and truly man, of a reasonable [rational] soul and body.” Thus this two-body teaching of Rome, which is an unavoidable implication of transubstantiation, is clearly an aberration and shows that the metaphors of the first Supper (“This is my body . . . This is my blood”) have been pressed “to the point at which they cease to be metaphors.” (Kenneth J. Collins and Jerry L. Walls, Roman But Not Catholic: What Remains at Stake 500 Years after the Reformation [Grand Rapids, Mich.: Baker Academic, 2017], 164)
Elsewhere, commenting on the dogma of the Immaculate Conception, they write the following:
[T]he elevation of Mary, declaring that she was conceived without a carnal nature, does indeed detract from the uniqueness and the dignity of Jesus Christ as the Savior of all humanity; nevertheless we also see something here that is far more basic and that therefore constitutes a direct affront to any sound Christology. That is, if Mary lacked original sin and therefore lacked a carnal nature, then she was like no other human being who has ever lived. In such a theological configuration, Mary is unlike Jesus because she was not divine, yet she is unlike the rest of humanity because she lacked original sin, a carnal nature. Thus she is not connected to Adam either as her federal head or as her origin. She is therefore unlike all the rest of humanity, in a class all her own.
What is being eclipsed here, in a roundabout sort of way, is the true humanity of Mary, and long with it nothing less than the true humanity of Christ as well. Once the latter is undermined, so is the unique status of Jesus as the God/Human. Since Mariological doctrines do indeed have christological consequences, the affirmation of the immaculate conception in effect denies that Jesus was truly human simply because he was not born of a woman who herself was really human, like the rest of humanity. (Ibid., 305-6)
