Why religious cannot agree.

[Angrboda]

Anybody have the numbers on people who change religions, and how many times they do this? Rationalizers may say these are not "true believers", but I think they're just samplers, looking for the one that best fits what they want religion to be.

This applies specifically to the U.S., so the pattern likely doesn't hold in general.

Like the 2007 Religious Landscape Study, the new survey shows a remarkable degree of churn in the U.S. religious landscape. If Protestantism is treated as a single religious group, then fully 34% of American adults currently have a religious identity different from the one in which they were raised, which is up six percentage points since 2007. If the three major Protestant traditions (evangelical Protestantism, mainline Protestantism and historically black Protestantism) are analyzed as separate categories, then the share of Americans who have switched religions rises to 42%.16 And these figures do not include an estimate of the number of “reverts” (people who leave their childhood religion before returning to it later in life). If the survey had measured this category, the estimates of the number of people who have switched religions would be higher still.

Along with other sources of change in the religious composition of the U.S. (like immigration and differential fertility or mortality rates), understanding patterns of religious switching is central to making sense of the trends observed in American religion. And perhaps the best way to assess the impact of switching on the composition of the U.S. religious landscape is to consider the ratio of the number of people who have joined each religious group to the number of people who have left. After all, every religious tradition ultimately loses some of the people who were raised within its fold, and every tradition (including the unaffiliated) gains some members who join its ranks after having been raised in a different group.

Looked at this way, the data clearly show that part of the reason the religious “nones” have grown rapidly in recent decades is that they continue to be the single biggest destination of movement across religious boundaries. Nearly one-in-five American adults (18%) were raised in a religion and are now unaffiliated, compared with just 4% who have moved in the other direction. In other words, for every person who has left the unaffiliated and now identifies with a religious group more than four people have joined the ranks of the religious “nones.”

By contrast, both Catholicism and mainline Protestantism, the two groups whose shares of the overall population have declined most sharply in recent years, have lost more members to religious switching than they have gained. Among U.S. adults, there are now more than six former Catholics (i.e., people who say they were raised Catholic but no longer identify as such) for every convert to Catholicism. And there are approximately 1.7 people who have left mainline Protestantism for every person who has joined a mainline denomination.

Pew Research Center || Religious Switching and Intermarriage

For the global pattern, Pew has this to say:

At present, the best available data indicate that the worldwide impact of religious switching alone, absent any other factors, would be a relatively small increase in the number of Muslims, a substantial increase in the number of unaffiliated people, and a substantial decrease in the number of Christians in coming decades.

Pew Research Center || The Changing Global Religious Landscape