Over 40 years out is a ridiculous amount of time to make these predictions. Christianity is rising fast in Africa and China. I do not know if it will continue for four decades.
Likely, but far, far from certain.
Christianity has also risen in Russia...
There is also no reason to give up on making America a Christian nation once again. There are many good Christians here who walk in Christ’s footsteps!
And I wonder if this optimism takes into account the growing intensity of muslim attacks in sub-Saharan Africa:
http://www.businessinsider.com/global-terrorist-attacks-past-20-years-in-maps-2017-5
None of these graphs seem to take into account the Holy Spirit, who works to will and to save. Christianity is not a business where you extrapolate tends. He is a person, He is very much at work, and He will save those who are His.