I'm certain being homogenous ethnostates has literally nothing to do with it. PS: a full third of Koreans believe in the God you're after, and almost all Japanese are religious of some sort.
The Japanese, Koreans, and Vietnamese also commit very little crime in the USA & Canada, two of the most diverse countries, so it doesn't matter if they're in a homogenous or heterogenous country or if they're fresh of the boat immigrants or are third generation citizens. They have low crime rates regardless.
Black Americans are the most Christian group in the USA and have the highest crime rates in the USA.
Around 31% of South Koreans are "Christian", but keep in mind that those include churches like the World Mission Society Church of God that has millions of members and goes door to door talking about worshipping the Heavenly Mother, an elderly lady that lives in Bundang, South Korea. Another church in South Korea is the Jesus Morning Star Church where the priest was sentenced to prison a while back for sex crimes and it's believed he may have had sex with upwards of 10,000 of his church women. And then there are the Moonies and so on and so forth. The Netflix documentary In the Name of God: A Holy Betrayal talks about the JMS church and a couple more churches in Korea.
www.netflix.com/title/81493078
In Japan, hardly anyone goes to churches or temples, or believes in God.
The Philippines is the Asia-Pacific country with high crime, and it has tons of churches and single moms.
Edit: You know those 24-hour unmanned stores that operate on the honor system in northeast Asia? Can you imagine opening one in Memphis?