Carrie Underwood made a surprise appearance at a religious conference on Monday night (Jan. 2) ... but not everyone's thrilled about it. The Atlanta pastor who founded the conference is facing backlash for Underwood's performance, because she has spoken out in support of gay marriage.

Readers can press play on the video above to watch Underwood's surprise performance of her hit "Something in the Water" at Passion 2017, an evangelical conference for college students, held at the Georgia Dome. The country star joined Christian singer David Crowder onstage, taking the 50,000-plus conference attendees completely by surprise -- a good one, judging by the crowds' cheers as Underwood begins the first lines of the song.

"What an incredible night @passion268!" Underwood tweeted after her performance, which occurred on the opening night of the three-day conference. "Thanks for letting me be a small part of it! & thanks @crowdermusic for letting me crash your set!"

However, as the Tennessean reports, Underwood's short set at Passion 2017 didn't go over well with some, including Wesley Wildmon, the director of outreach for the American Family Organization. In Engage magazine, a publication run by the Christian non-profit organization and for which Wildmon is social media coordinator, Wildmon published an open letter to Rev. Louie Giglio, who founded the Passion conference, voicing his discontent.

"I was very frustrated that you would allow her to help lead thousands of people in worship. My frustration quickly turned to disappointment and then to sadness," Wildmon writes. "Carrie Underwood encourages and supports homosexual marriage, which the Word of God does not (1 Corinthians 6:12-20) ... With the many Christian artists who believe and teach the full counsel of God’s Word available to lead worship at Passion, why would you choose one who publicly states homosexuality is not a sin? ... Your approval gives thousands of unsaved or weak Millennials an untruthful image of what the Bible says about marriage."

This is not the first time that Underwood has faced criticism for her support of gay marriage; in fact, ever since she first publicly voiced her support in 2012, she's received backlash.

“Someone asked me a question, and I tried to give an honest answer,” Underwood told Redbook in 2013. “If someone won’t like me because of that, then maybe they were looking for a reason not to like me.”

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