The total population of the unaffiliated is projected by Pew to grow three percent over this time period, but the world's population is projected to grow 32 percent. Christians will hold their 2015 percentage, 31% and slightly increase it, the Muslims who were at 24% in 2015 will be about equal with the Christians in 2060 according to Pew.
You can read the Pew article "The Future of World Religions" here. Pew provides you with their estimates of what will happen, the rest of this page will provide you with my views of what may push the numbers higher or lower over the next few decades.
When the Soviet Block fell the Russians did not all immediately rush back to religion but over the last few decades the Russian Orthodox Church has reasserted itself. By some statistical measures Russia is now about as religious as the United States. If the Communist Party loses power in China something similar could happen there. The future of China is uncertain and this introduces a lot of uncertainty into what percentage of the world's population will be religiously affiliated and what religion they will be affiliated to.
What has remained predictable is that religious people have far more babies than the unaffiliated, and the unaffiliated do not have enough children to reproduce themselves. According to Pew the religiously unaffiliated have 1.6 children per woman, the Christians 2.6, the Muslims 2.9 and all religions 2.4. This is crucial to the falling percentage of the unaffiliated. Nevertheless the unaffiliated are expected to grow in absolute numbers, the conversion of religious people to unaffiliated status is no doubt part of this as the unaffiliated are no where close to having enough children to break even.
Developed nations, like the unaffiliated tend to have low birth rates, but in recent years this has been more than made up by the net migration of people from developing to developed nations and national economies ascending from developing to developed status.
It is frequently argued that as people reach a comfortable level of income they will tend to lose their religious faith. There is considerable evidence for this, but it could turn out that relative income rather than absolute income is more important.
Religions often emphasize the responsibility of the rich to share their wealth with the poor. This idea is likely to be more popular among the relatively poor than the relatively rich. The relatively rich are not likely to find what Jesus said about it being easier for a camel to pass through the eye of a needle than for a rich man to enter the kingdom of heaven comforting. So as the developing countries enter into high income or developed status their people may not consider themselves rich as they will still be relatively poor. Thus the people of these new developed countries may remain religious.
Mother Theresa provides an example. She felt it necessary to live as one of the poorest of the poor. As poverty is reduced and the poorest of the poor become more comfortable living as one of the poorest of the poor will be less of a sacrifice. This may make religion more popular.
However, the focus of conflict is shifting away from Islam and toward China and North Korea. Where before religious fanatics were the enemy and religion was suspect, now Communists are once again the enemy and atheists therefore may again become suspect as they were in the cold war. The cold war may have strengthened religion in America, and may have been one of the reasons that America was so religious compared to other developed democracies. This might happen again.
On the other hand the Soviet threat may have discouraged religion in Europe. Europeans adopted economic policies, for example socialized medicine, and relatively generous help for the poor that weakened the communist critique of capitalism. They accommodated the communists. Perhaps the weakening of religion in Europe was part of the same accommodationist trend.
This might suggest that those countries that are closest to China might see reductions in religious belief, but perhaps those that are far away, for example, Europe might go in the opposite direction.
On the other hand the Internet has also allowed the atheists to more widely broadcast their message. At this point the Internet seems to be a win for secularists.
In the future artificial intelligence can, like the Internet, be seen as force for which ever position you believe in. Artificial intelligence could theoretically act as a neutral observer, having neither a desire for an afterlife nor a fear of hell. The artificial intelligence would not reject a religious message because it did not want to give up its sins. What sins could a machine have. So both secularists and religious believers might look forward to the artificial intelligence supporting them just as both secularists and religious believers may believe the Internet will favor them.
If you wish to read more of my speculation about artificial intelligence and religious belief you can check out some other links on the economics index page, particularly the one where I explore a senario where artificial intelligence convinces the human race to vote itself into extinction. and the Devil's History of Life and Humanity.
Tell me what you think. Here is my contact information..Posted May 24, 2020
World Religious Statistics How many believe, organized by similar beliefs.
Growth prospects for Islam
Growth prospects for Islam in the 1st World
Growth prospects for Christianity