Have we seen a large population "transformed in a day" in recent history?There are no countries that were under 5% Christian in 2005, and over 5% Christian in 2015. The World Christian Database tells me there are only 6 countries that were "under 5% Christian in 1970, and over 5% Christian in 2015" (30 year change). Three of these were church growth stories, and three are simply overseas foreign workers:
There were, by contrast, 43 changed in a century: under 5% in 1900, over 5% in 2015. Most of these were dramatically so (e.g. under 5% in 1900, over 60% in 2015):
Sustained growth over decades is important to see national transformation. Countries that were less than 1% Christian in 1900 now widely have the Gospel. Persistence and multiplicative growth are key.