From the so-called Athanasian Creed:
Furthermore, it is necessary to everlasting salvation : that he also believe rightly the Incarnation of our Lord Jesus Christ.
For the right Faith is, that we believe and confess : that our Lord Jesus Christ, the Son of God, is God and Man;
God, of the substance of the Father, begotten before the worlds : and Man of the substance of his Mother, born in the world;
Perfect God and perfect Man : of a reasonable soul and human flesh subsisting.
Equal to the Father, as touching his Godhead : and inferior to the Father, as touching his manhood;
Who, although he be God and Man : yet he is not two, but one Christ;
One, not by conversion of the Godhead into flesh : but by taking of the Manhood into God;
One altogether; not by confusion of Substance : but by unity of Person.
Christ is God and Man by the taking of manhood into God; likewise his Body, the Church, is taken up from her humanity into the communion of his divinity.
Make no mistake, we have made progress. The catholic faith whole and entire, the Holy Spirit has deigned to make the Church grow into her head, into Christ, from whom the whole body, joined and held together by every joint with which it is equipped, when each part is working properly, makes the body grow so that it builds itself up in love. Except the Church is still apart in schism and heresy: this is our ugly, our centuries-long disease. But God’s will is that we be together once more, caught up in a holy romance.
Our schism, our failure to be united in the pure catholic faith, is practical heresy for its misrepresentation of God’s substance. The Son is of one substance with the Father; is the Church of many substances, many Lords, many faiths, many baptisms? Forbid it, Lord!