The #Pew Research Center has a report published in 2015 about the future of population of religions that is interesting:
https://www.pewresearch.org/religion/2015/04/02/religious-projections-2010-2050
And also following article is about this report:
https://www.npr.org/sections/goatsandsoda/2015/12/25/460797744/a-religious-forecast-for-2050-atheism-is-down-islam-is-rising