what will replace Christianity most likely will be agnosticism and atheism ... not a good alternative.
Humanism is a better ideology than Christianity and has been since the Enlightenment.
Humanism promotes reason and evidence over faith, tolerance over irrational moral judgments, and rational ethics based in the Golden Rule and utilitarianism rather than ancient received moral codes. Humanism accomplishes what Christianity only promises - societal well-being. Humanism gave us science and the modern liberal democratic state with guaranteed freedoms. Humanism informed Christianity that slavery and pedophilia are immoral, but homosexuality and atheism not. Humanism converted astrology and creationism to astronomy and evolution science. Humanism converted serfs and subjects to middle class autonomous citizens. What was Christianity's last contribution to elevating the human condition? What was Christianity's last idea that bore fruit?
People are finding Christianity increasingly unkind, irrational, ineffectual, and irrelevant.
I think it will be a lot sooner than 2070. More like 2040.
Agree. I did this calculation myself over a decade ago based in Pew and ARIS data, and came up with 2039.
This is the ARIS data from 2009:
Total Christian
1990: 151,225,000 (86.2%)
2008: 173,402,000 (76.0%)
From Pew in 2014, we learned that people self-identifying as Christians fell further to 70.6% in 2014
These are different sources probably using slightly different methodologies, but we can still estimate the rate of this decline from this data. We had a 10% fall in 18 years (about 0.55% per year), then 6% more in six years.
2039 is 25 years later (1% per year). The exodus from Christianity is accelerating as we would expect as atheism became more socially acceptable, but even were it to remain 1% per year, that's a fall to about 45% by 2039. I should probably change that estimate 2034-5.
Yet it is Pew predicting 2070.