Churches should be contextualized, I agree. But the gospel always challenges context. There must be a healthy balance between embracing and exploding context. A church should never be a church for only one kind of person. The gospel isn’t anti-culture, but neither does it enthrone any single culture. The gospel transforms culture.
It is one thing to begin a church with members from a single culture, but if that church never preaches a gospel that challenges the culture then it isn’t really preaching the gospel.
People groups were God’s idea, but God is also about having one People with One Shepherd. I just read a chapter by a church planter who veers dangerously close to giving the idea that God likes segregation. He argues that people groups were God’s idea from the passage about Babel in Genesis 11. While it is true that God confounded the people who had become idolaters of their own culture, all of the scriptures from that moment on move towards the re-establishment of one people under one shepherd. The structure and message of all of the prophetic books is seen in woes pronounced on Israel, woes pronounced on the nations, followed by a restoration in which the nations are brought back into a single flock with Israel. Jesus, himself, quoted from Ezekiel in John 10 when he talked about other sheep who were not a part of the flock of Israel who he had to bring into the fold.
I want to be balanced and fair to those who are writing about context, but if your theory about how we should contextualize begins and ends in Genesis, start over. A better example would be Acts 16 in which Paul starts the church in Philippi with a woman (Lydia), a possessed slave girl, and a Roman jail keeper. The model that Acts gives us is that while we may contextualize the gospel based on continent, or even region the truth is that the kingdom of God is a migrant kingdom made up of every tribe and tongue.