Joe Carter says it makes more sense to look at long-term trends:
In a recent interview in which she announced she had joined the Episcopal Church, Rachel Held Evans said,
Just about every denomination in the American church— including many evangelical denominations — is seeing a decline in numbers, so if it’s a competition, then we’re all losing, just at different rates.
Many Americans, both within and outside the church, share Evans perception of the decline of denominations. But is it true? Are most denominations truly seeing a decline in numbers?
Before we answer the question, we should clarify what is meant by “decline.” We could, for instance, say that Protestantism has been on the decline since the 1970s. That would be true. We could also say there are now more Protestants today than there were in the 1970s. That too would be true.
The fact is that the percentage of people identifying as Protestant has declined since the 1970s while the total number of Protestants has increased (62 percent of Americans identified as Protestant in 1972 and only 51 percent did so in 2010). Yet because of the population increase in the U.S., there were 28 million more Protestants in 2010 than in 1972.
So did Protestantism in America decline since the 1970s? Yes (percentwise) and no (total numbers).
What about when we drill down to the denominations that comprise Protestantism in America? Here the differences depend on whether we look at short-term or long-term trends.