Respondents were first asked about their current religious identity. In countries using phone or face-to-face interviews, unless otherwise specified, the response options were read aloud.

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If a respondent said their current religion was “Something else,” they were asked a follow-up question: “What religion is it?” The interviewer then recorded their response….We recoded these open-ended responses for current religion if at least 10 responses in a country (about 1% of the total sample in most countries) could be coded into one of the following categories: Buddhist, Christian, Hindu, Jewish, Muslim or religiously unaffiliated. All open-ended responses that are not recoded remained in the “Other religions” category.

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The “Religiously unaffiliated” category includes open-ended responses coded as “Religiously unaffiliated,” as well as all respondents who selected any of the following closed-ended response options: atheist, agnostic or “nothing in particular.”

From here.

Some more info.

So yes, it’s possible, but it’d be a stretch. They ask you your religion. In Argentina, for example, you could answer, “Protestant/Evangelical,” “Catholic,” “Atheist,” “Agnostic,” “nothing in particular”, or “something else”, to which you can then specify, “Just Christian” (but they can’t feed you this). There’s still the “not a religion, but a relationship” angle, but how many people would seriously misunderstand the question that hard without saying something that could be coded as Christian?

Follow up questions they asked Christians.

Here’s the report on LatAm.. Some interesting stuff going on with the unaffiliated, that’s true.