HOT SPRINGS, S.D. — In the past couple of years, conservative opposition to same-sex marriage has clearly started to erode. Prominent Republicans like Senators Rob Portman and Lisa Murkowski and former Secretary of State Colin L. Powell have come out in support of gay marriage. Even David Blankenhorn, the expert witness in the Proposition 8 trial in California and a Democrat, announced that he had changed his mind.

They are, for the most part, moderate conservatives using secular, democratic arguments. None come from the Christian right. Among religious conservatives, opposition to same-sex marriage has remained essentially unquestioned.

Which is why “The Things We Share: A Catholic’s Case for Same-Sex Marriage,” an essay by Joseph Bottum, published Friday on the Web site of Commonweal magazine, is something new in this debate.

Mr. Bottum, 54, is a serious Christian. He attended Roman Catholic high school, college and graduate school. His erudite writing for conservative magazines like National Review and The Weekly Standard is laced with references to church history and theology and to Christian writers like G. K. Chesterton and W. H. Auden. He fiercely opposes abortion, and for five years, until 2010, he was editor in chief of First Things, a key opinion journal for religious conservatives.