The graph below presents countries plotted by estimated 2025 population (UN Population Prospects, 2018 edition) by estimated 2025 % Christian (my own data, projected based on data from the Atlas of Global Christianity). Here, by Christian, we are referring to the widest possible definition.
The graph's % Christian tick marks are divided according to my Stages of Christianity, at the 2% (Stage 2), 8% (Stage 3), 32% (stage 4), and 90% Stage 5. Stage 1, less than 0.01%, is not indicated because my initial drafts caused too much crowding on the map, and it wasn't all that useful at this level of analysis.
Colors on the graph are chosen to represent four stages of growth.
Finally, the map below shows which countries are growing fastest according to the four stages of growth.
There are three things I think worthwhile to explore about this data:
1. The population of most countries ranges between 1 million and 100 million. Only a handful of countries are more than 100 million in size, and obviously only 2 are over the 1 billion mark at the very top.
2. Countries with fewer than 1 million people tend to be heavily Christian: around the 90% Christian mark. In these places, Christianity is growing at rates smaller than population growth: Christianity is losing its percentage share. But this is a fairly slow process, and they will likely be heavily Christian for the forseeable future. Slower growth rates are to be expected: most individuals within such communities are born into Christianity, and any conversion choice by default is to defect out. Individuals born into non-Christian communities in such situations are in an extremely marginalized situation (very small diaspora communities, or collections of agnostics/atheists) and are not as likely to convert into Christianity.
3. Countries over 1 million in size are very likely to see growth in Christianity outpacing the general population, with two exceptions. The first: countries that are less than 2% Christian are typically seeing Christianity losing numbers or at least share (although there are many "blue" rapid population dots in that side of the graph). Second, heavily-Christian countries have a higher number of losing-share countries (on the graph, countries in this category appear to be about half growing, half losing).
Right now, most of the Christian growth is occurring in low-% Christian, 1 million plus populations, especially those within about 8 to 80% Christian.