In a fleeting Instagram story in early August, the pop star Britney Spears announced that she had just returned from mass and was "a Catholic now." Fans immediately recognized this faith as a departure for Spears, who was raised Baptist.

The Instagram post was deleted within a day of its appearance, but not before eliciting congratulations from fellow Catholics and incredulous commentary on Twitter with some asking whether the singer — who dressed up as a Catholic schoolgirl in the provocative video for "...Baby One More Time" (1999) was now "really" a Catholic.

This is not surprising. The religious choices of American celebrities have long invited incredulity. And it's not just pop culture curiosity. This fascination with Spears's religion highlights an ongoing conversation within American culture about authentic identity; something religious affiliation has long been used to determine.

Consider, for example, the doubt and derision that Sammy Davis Jr. faced when he started telling friends and fans that he was Jewish in the late 1950s. Davis was among a small cohort of Black performers who broke through the segregated entertainment industry after World War II to find success with white audiences. Following a near-fatal car accident in 1954, Davis resurrected his career as a singer, impersonator, dancer and actor. Fitted with a glass eye to replace the one destroyed in the accident, Davis was irrepressibly energetic. The source of his renewed confidence, he soon explained, was Judaism.

Fellow comedians — many Jewish — roasted Davis for combining Blackness and Jewishness. At Davis's bachelor party in 1960, Milton Berle, in drag, performed a parody, "My Yiddish Mau Mau," while Peter Lawford sang, to the tune of "The Lady is a Tramp," "That's Why That Sammy is a Jew." Eager to show he was in on the joke, Davis wore out punchlines about the seeming incongruity of his identities. His golf handicap, he deadpanned, was being "a one-eyed Negro who's Jewish."

African-American Christians leveled harsher criticism in the pages of Black-owned magazines such as Ebony, questioning whether Davis had abandoned Blackness when he chose Judaism. One fellow entertainer suggested the conversion was a "gratuitous" effort to curry favor with Jewish nightclub owners and audiences.

But Davis could not have been more earnest about his Jewishness. He considered it a natural extension of his civil rights activism. Nothing could be more logical, he argued, than to find common themes between the historic struggles of Jews against anti-Semitism and the contemporary challenges facing African Americans in a segregated nation.

These commitments to Jewish and Black identity shaped Davis's politics. As a visible and generous participant in the civil rights movement, Davis sent airplanes filled with celebrity activists and hefty amounts of cash to support protesters and organizers, and he marched alongside Martin Luther King in Selma in 1965. Davis's philanthropy likewise extended to American Jewish and Zionist organizations. His fervor matched that of many American Jews, who celebrated Israel's military victories and territorial claims following the 1967 war with its Arab neighbors.

In his praise of Israel and its military and his support for Richard M. Nixon's reelection in 1972, Davis gradually adopted a Jewish politics more aligned with White conservatism than with the mainstream African-American civil rights movement.

At the end of his life, Davis seemed resigned to the fact that his dual identities as Black and Jewish struck most audiences as surprising and even laughable. He created a shtick in which he reminded his audience of his conversion decades earlier and of the prejudice he confronted as a result.

"It's very serious," he said of his conversion. "Of course, I've had to fight humor with humor."

Years of heavy smoking, drug use and alcoholism took their toll, but Davis continued to perform in front of appreciative Jewish nightclub crowds well into the 1980s.

A minister and a rabbi spoke at Davis's funeral in 1990, the minister probably there at the request of Davis's non-Jewish wife Altovise.

Davis’s journey provides lessons to help us understand the role of religion in public life. It reveals deep-seated biases about religious credibility. Whose choices are authentic, and whose are "gratuitous?" Public interest in religious conversion has declined over the last three decades, but conversions that challenge expectations about racial identity or sexuality remain controversial. Given how often Americans ground their politics in declarations of sincere faith, we should recognize the political significance of attempts to delegitimate a convert's religious, racial, or sexual self-expression.

Spears's announcement received even more media attention than it might have otherwise because she is in the midst of challenging the legal conservatorship that until recently kept her dependent (financially and otherwise) on her father and his team of lawyers. As part of her case to end the conservatorship, Spears testified that her father would not allow her to remove an IUD or make choices about her reproductive options. Some Roman Catholics have leaped to her defense as a case of coercive birth control.

Surprise at Spears's conversion reflects this longer history of doubting whether certain celebrities possess the moral seriousness to "truly" adopt a new faith. Both Roman Catholicism and Judaism require converts to learn their adopted faith's liturgy and core values. No matter how many times Davis spoke about the Talmud on his bedside table or his appreciation for Jewish history, his chosen religious affiliation struck his contemporaries as more of a punchline than anything else. In the social media chatter that followed Spears's Instagram announcement, doubts circulated of whether she could have possibly completed the Rites of Christian Initiation for Adults, a months-long course.

Doubts about whether a Black entertainer can "really" be Jewish or whether a hyper-sexualized female pop star can "really" become Catholic get at the heart of long-standing debates over the politics of authenticity.

Ultimately, if Spears tells us she is a Catholic, who are we to judge?

Rebecca L. Davis is the author of "Public Confessions: The Religious Conversions that Changed American Politics."