The Latest Religious Affiliation Trends In The U.S.

Pew Research reports: 

The Center estimates that in 2020, about 64% of Americans, including children, were Christian. People who are religiously unaffiliated, sometimes called religious “nones,” accounted for 30% of the U.S. population. Adherents of all other religions – including Jews, Muslims, Hindus and Buddhists – totaled about 6%. 

Depending on whether religious switching continues at recent rates, speeds up or stops entirely, the projections show Christians of all ages shrinking from 64% to between a little more than half (54%) and just above one-third (35%) of all Americans by 2070. 

Over that same period, “nones” would rise from the current 30% to somewhere between 34% and 52% of the U.S. population. . . .

As recently as the early 1990s, about 90% of U.S. adults identified as Christians. In 2007 the share was at 78%. Today, that number is down to 64%. Since 2007, the share of adults who identify as religious “nones” has grown from 16% to 29%.

A mid-range estimate is 50% Christian, 5% other religions, and 45% "none" by 2070, which would be a 50% increase in the proportion of "nones" and a 12% decrease in the proportion of Christians. This might even involve a net decline in the absolute number Christians because population growth in the United States is leveling off in the same time frame.

Changes in religious affiliation tend to follow an S-shaped logistic curve, with rates of change highest when the proportions are each category are most similar, and lowest when all but one of the main categories is very small (e.g. single digit percentages), so a mid-range estimate based upon current rates of change in religious affiliation is more likely to be an underestimate than an overestimate.

Put another way, the 2070 midrange estimate is likely to actually arrive well before 2070.