God defines marriage. To accept same-sex marriage is heresy.

o begin, I should state how the Church defines heresy. You can find the def­i­n­i­tion in Canon 751 as well as the Cat­e­chism of the Catholic Church §2089:

Heresy is the obsti­nate denial or obsti­nate doubt after the recep­tion of bap­tism of some truth which is to be believed by divine and Catholic faith.

Not just “Catholic” faith, but “divine” faith too. Canon 750 tells us what sorts of things con­sti­tute “divine and Catholic faith.”

A per­son must believe with divine and Catholic faith all those things con­tained in the word of God, writ­ten or hand­ed on, that is, in the one deposit of faith entrust­ed to the Church and at the same time pro­posed as divine­ly revealed either by the solemn mag­is­teri­um of the Church or by its ordi­nary and uni­ver­sal mag­is­teri­um.

These divine truths require “com­mon adher­ence.” “All are bound,” the canon says, “to avoid any doc­trines what­so­ev­er con­trary to them.”

So a heresy involves more than just a denial of what the Church pro­pos­es to be true. To speak heresy, one must also deny what God has revealed to be true.

But does God define mar­riage in his rev­e­la­tion?

•••

Canon lawyer Edward N. Peters argues that he does. In the wake of the SCOTUS deci­sion in Oberge­fell v. Hodges, Dr. Peters wrote these words on his canon law blog:

If, as seems like­ly, Church teach­ing that mar­riage can exist only between a man and woman is taught not just infal­li­bly (as a ‘sec­ondary object’ of infal­li­bil­i­ty) but as being divine­ly revealed (mak­ing it a ‘pri­ma­ry object’ of infal­li­bil­i­ty), then, a Catholic’s obsti­nate denial of such a truth is canon­i­cal­ly “heresy” (Canon 751) pun­ish­able by excom­mu­ni­ca­tion (Canon 1364 § 1), an auto­mat­ic one at that—and is not just ‘oppo­si­tion to Church teach­ing’ pun­ish­able by a ‘just penal­ty.’

The bib­li­cal basis for the argu­ment is Matt. 19:4. The Phar­isees asked Christ whether it were law­ful for a man to divorce his wife “for every cause.” Christ in turn asked them, “Have ye not read, that he which made them at the begin­ning made them male and female?” In this verse, Christ is quot­ing Gen. 1:27.

In order to deny that Church teach­ing on this mat­ter is a point of divine rev­e­la­tion, one would have to claim either or both of two things:

• that the words of Christ in Matt. 19:4 are not divine rev­e­la­tion;
• that Matt. 19:4 does not apply to the ques­tion of same-sex mar­riage.

I do not see how one could sus­tain either claim.

1. Christ is God, and what­ev­er he teach­es is, by def­i­n­i­tion, divine rev­e­la­tion.

2. Christ is answer­ing a ques­tion about divorce; but to claim that he does not stat­ing a prin­ci­ple that applies to any ques­tion about mar­riage, is arbi­trary and unnec­es­sar­i­ly lim­its the text.

Every time the Church speaks about same-sex marriage—every time—it grounds its objec­tion in Gen. 1:27 and Matt. 19:4.

Here, for exam­ple is how the Con­gre­ga­tion for the Doc­trine of the Faith dis­cuss­es the point. The title of the doc­u­ment is “Con­sid­er­a­tions Regard­ing Pro­pos­als to Give Legal Recog­ni­tion to Unions Between Homo­sex­u­al Per­sons.” (And yes, that includes civ­il law too.)

The nat­ur­al truth about mar­riage was con­firmed by the Rev­e­la­tion con­tained in the bib­li­cal accounts of cre­ation, an expres­sion also of the orig­i­nal human wis­dom, in which the voice of nature itself is heard.

This is a nat­ur­al law argu­ment, but not just a nat­ur­al law argu­ment. The CDF says that the truth is “con­firmed by Rev­e­la­tion.” It is “in the bib­li­cal accounts.” Those who com­plain about the Church’s reliance on nat­ur­al law will need to note this. The Church does not start with nat­ur­al law and stop there.

There are three fun­da­men­tal ele­ments of the Cre­ator’s plan for mar­riage, as nar­rat­ed in the Book of Gen­e­sis.

In the first place, man, the image of God, was cre­at­ed “male and female” (Gen. 1:27). Men and women are equal as per­sons and com­ple­men­tary as male and female. Sex­u­al­i­ty is some­thing that per­tains to the phys­i­cal-bio­log­i­cal realm and has also been raised to a new level—the per­son­al level—where nature and spir­it are unit­ed.

Mar­riage is insti­tut­ed by the Cre­ator as a form of life in which a com­mu­nion of per­sons is real­ized involv­ing the use of the sex­u­al fac­ul­ty. “That is why a man leaves his father and moth­er and clings to his wife and they become one flesh” (Gen. 2:24).

Third, God has willed to give the union of man and woman a spe­cial par­tic­i­pa­tion in his work of cre­ation. Thus, he blessed the man and the woman with the words “Be fruit­ful and mul­ti­ply” (Gen. 1:28). There­fore, in the Cre­ator’s plan, sex­u­al com­ple­men­tar­i­ty and fruit­ful­ness belong to the very nature of mar­riage.

The CDF, like Christ, says that the cre­ation of human beings “male and female” was part of God’s design for mar­riage, and it calls this a point of rev­e­la­tion. It is not just some­thing the Church pro­pos­es; nor is it nat­ur­al law alone. It is also a truth revealed by God.

That would put denial of this truth into the cat­e­go­ry of a heresy.