I just made this map. Is it mostly correct? For the dark blue sections, I only included countries where Christianity is still relevant, even if it's only to a minority of the population, as well as the majority of course. So an obvious answer for the majority would be the United States or Poland. But a strong, religious Christian minority example would be in Lebanon (35% or so) and Egypt (15%). Countries like Spain or Britain may have more Christians in bulk than Lebanon or Egypt, but the actual number of religious Christians is miniscule there, and they are no longer relevant. I debated whether or not France should be included, you guys tell me.
The light blue sections consist of countries where Christianity is growing rapidly.
> China
At least 70million Christian converts from Taoism or atheism. Enormously powerful movement that is being persecuted by the communist Chinese government, very sadly nostalgic of what the Soviets did to orthodox churches. Still, their growth is unstoppable in China, making it the fastest-growing religion in the world's most populous country.
> Indonesia
While less populated than China, Indonesia actually has the fastest growth rate of Christian converts. It's the most populated Muslim nation on earth, but because there is no punishment for conversion/apostasy from Islam, the conversion rate is enormous. By 2040, it is possible that the world's largest Muslim nation could be majority Christian.
> Iran
Unlike Indonesia, apostasy from Islam is punishable with imprisonment, lashing, and death in some cases. Yet Christianity is still the fastest-growing religion in the Islamic Republic. There was already an existing minority of ancient Persian christians, but recently millions more have shown interest. The actual numbers in Iran are unknown due to the Islamic government refusing to do a legitimate census. Most of these new Iranian christian converts practice underground.
> India
Most people don't think of India when they think of Christianity, but there is actually a decent population. Like Iran, India always had a tiny Christian minority dating back to ancient periods, but there is a larger conversion movement in the past 15 or so years.