And last month, Trump criticized Pope Francis for questioning his faith, though the real estate mogul apparently doesn't hold himself to the same standard.
Trump’s campaign did not immediately respond to ABC News’ request for comment.
"He needs to act like he's religious to appropriate groups," Shah told ABC News. I think it's pure opportunism and, in that sense, I don't think it's the least bit surprising. ... What's surprising is that so many people fall for it."
Shah was among 37 Catholics who signed an open letter against Trump that was published in the National Review.
Shah noted that Trump only started talking about his own faith after he started running for president, but that wasn't the start of businessman’s questioning political leaders' religion.
Trump led the birther movement questioning President Obama's birthplace, suggesting in 2011 during an interview with Fox News that "maybe it [Obama’s birth certificate] says he is a Muslim." It doesn’t.
Trump has also gone after at least one of his campaign rivals: Sen. Ted Cruz. Citing the controversy over alleged calls by Cruz's campaign saying that Ben Carson was dropping out of the race shortly before the Iowa primary, Trump called Cruz a liar.
"I've never seen anybody that lied as much as Ted Cruz ... and he goes around saying he's a Christian," Trump said in February.
At a different rally that same week, Trump said Cruz "holds up the Bible and then he lies about so many things."
Trump may still be raising the religion question partly because it appears to be resonating with certain groups of evangelical Christians, Shah says.
"That's appealing to evangelicals who are feeling marginalized by a political culture that they see as detesting them or marginalizing their faith as a result of political correctness," Shah said.
Others, including a group of rabbis and Jewish leaders that are planning to protest his speech at a Jewish summit today in Washington, believe his rhetoric that they say divides the population into "us" and "them" is encouraging discrimination.
"I think that Mr. Trump's approach is a nationalist approach, and it's that there's an 'us' and a 'them,'" Rabbi David Paskin told ABC News.
"For us Jews, we believe that there's one God, and there's one God that created all of us and that means that we are inextricably linked to one another. ... [Trump's] approach to this whole campaign is an anathema to my faith," Paskin said.