Christians rejected the biblical advocacy of slavery. Christians have (for the most part) rejected the Bible's statements on the inherent inferiority of women. Christians have (for the most part) rejected the Bible's call to genocide against the unfaithful. And for years, progressive Christians have rejected the culturally-conditioned prejudices of the biblical writers on homosexuality. (See for example, my column on President Obama's recent theological reasoning for gay marriage.)

Now, many conservative Christians, particularly young men and women who have grown up among openly gay friends and older men and women who love their gay children and grandchildren, can no longer reconcile the harsh words of the Bible with their experience of gay people as fully human and equally beloved by God.

I know this journey well; in the course of my life, I have gone from a homophobic conservative Christian raised in a small town to a person who recognizes that the gay men and women I have known are often more faithful to God and to those they love than I have been.

If I can change—this country boy from Oklahoma, who once thought being called a "fag" was the most heinous insult possible, the one thing for which it was worth getting beat up—then believe me, all of us can change.

And we should all change—because it is the right thing to do.

Republicans should be on the right side of history—and not just because gay marriage is a voting issue.

They should be on the right side of history because it is the right side of history.

Republicans were right about slavery: wrong.

Republicans were right about denying women the vote: wrong.

And they have the opportunity to be right again about something that is wrong—even though it will mean a radical shift in position, and will alienate some members of the Republican base.

And that, frankly, is a good thing.

As I wrote in Faithful Citizenship, the frantic partisanship of our parties and adherence to our own beliefs has blinded us to the possibility of engaging with those who differ from us, from learning from each other. The tendency of both major parties to pander to their radical base for fear of being "primaried" come next election has to change. What is good for some gerrymandered district back home is not good for the nation at large, nor for the larger world.

I do not insist that you personally agree with me on gay marriage. If your faithful reading of the Bible, if your internal survey of your own conscience prohibits your support on this issue, I will sadly disagree with you—but I will still love you.

But I do ask that you not insist that your entire party—that our entire nation—conform to your beliefs.

The arc of history has finally turned, and any political party that wants to remain relevant and be perceived as just had better change its policy on gay marriage.

Because you will lose votes for opposing gay marriage.

Because younger voters and black, Hispanic, and Asian voters will turn away.

Not to mention the fact that it's wrong.