IV. All heresies proceed from failure to believe the twofold nature of Christ
The catholic Faith then, dearly beloved, may scorn the errors of the heretics that bark against it, who, deceived by the vanity of worldly wisdom, have forsaken the Gospel of Truth, and being unable to understand the Incarnation of the Word, have constructed for themselves out of the source of enlightenment occasion of blindness. For after investigating almost all false believers' opinions, even those which presume to deny the Holy Spirit, we come to the conclusion that hardly any one has gone astray, unless he has refused to believe the reality of the two natures in Christ under the confession of one Person.
For some have ascribed to the Lord only manhood, others only Deity. Some have said that, though there was in Him true Godhead, His flesh was unreal. Others have acknowledged that He took true flesh but say that He had not the nature of God the Father; and by assigning to His Godhead what belonged to His human substance, have made for themselves a greater and a lesser God, although there can be in true Godhead no grades: seeing that whatever is less than God, is not God. Others recognizing that there is no difference between Father and Son, because they could not understand unity of Godhead except in unity of Person, have maintained that the Father is the same as the Son: so that to be born and nursed, to suffer and die, to be buried and rise again, belonged to the same Father who sustained throughout the Person of both Man and the Word.
Certain have thought that our Lord Jesus Christ had a body not of our substance but assumed from higher and subtler elements: whereas certain others have considered that in the flesh of Christ there was no human soul, but that the Godhead of the Word Itself fulfilled the part of soul. But their unwise assertion passes into this form that, though they acknowledge the existence of a soul in the Lord, yet they say it was devoid of mind, because the Godhead of Itself was sufficient for all purposes of reason to the Man as well as to the God in Christ.
Lastly the same people have dared to assert that a certain portion of the Word was turned into Flesh, so that in the manifold varieties of this one dogma, not only the nature of the flesh and of the soul but also the essence of the Word Itself is dissolved.
Source: Sermons (New Advent)