A study published by the Institute for the Study of Societal Issues says that the number of people who claim no religious affiliation as grown. Most of these "nones" are people who believe in God and most had a religious affiliation earlier in life.
Here is the abstract and the rest of the study:
Twenty percent of American adults said in 2012 that they had no religious preference, according to the latest General Social Survey (GSS), a nationally representative survey of American adults. This continues a trend of Americans disavowing a speciﬁc religious afﬁliation that began in the 1950s but has accelerated greatly since 1990. The GSS has asked adults the following question for forty years: “What is your religious preference? Is it Protestant, Catholic, Jewish, some other religion, or no religion?” The percentage answering “no religion” was 18 percent two years earlier in 2010, 14 percent in 2000, and 8 percent in 1990. The upward trend in the “no religion” choice is very broad. While some types of Americans identify with an organized religion less than others, Americans in almost every demographic group increasingly claim “no religion” since the trend began to accelerate in 1990. Preferring no religion is not atheism which is still very rare; in 2012, just 3 percent of Americans said they did not believe in God. Comparing religious origins with current religion we ﬁnd that while 20 percent of adults currently have no religious preference, only 8 percent were raised without one. The GSS is an especially important source, because it conducts 83 percent of its interviews in person and has an uncommonly high response rate of over 70 percent.
From the study:
The GSS asks people who expressed a religious preference “What speciﬁc denomination is that, if any?” People have named over 300 speciﬁc denominations in reply. Scholars classify those responses in various ways. Here we distinguish conservative Christian responses like Baptist, Christian, Pentecostal, and Church of God in Christ from other Protestant denominations such as Episcopal, Methodist, Lutheran, and Presbyterian among the Protestants; we also distinguish Catholics and Jews from other religions. The same classiﬁcation applies to religious origins and current religions.
About one-third of Americans were conservative Christians in 2012, and about one-third were raised in a conservative Christian tradition. There is lots of ﬂux behind that unchanging number as new converts to conservative Christian churches offset defections by others, resulting in little change in the size of the conservative Christian segment within the generation.
The same is more or less true of the mainline Protestant denominations. While 19 percent of adults in 2012 were mainline Protestants 21 percent were raised in that tradition.
The Catholic church experienced the greatest net exit within the generation. One-fourth of American adults were Catholic in 2012. If we were to compare that share with the current religion of Americans in the past, we would think there had been little change. Throughout the last fifty years, a consistent 25 percent (plus or minus one percentage point) of Americans were Catholic