I mean, (the fictional nation of) Wakanda is depicted as a very isolationist country, and it's reasonable to expect that Christianity wouldn't have taken root if foreign missionaries were expelled throughout its history. Tokugawa-era Japan may be the closest comparison, where its isolationist policies resulted in fairly high retention of indigenous religions even through to modern times. Today less than 2% of Japan's population is Christian, whereas nearly 30% of South Koreans are Christian.