I think Robert Frost was on to something when he wrote, "Something there is that doesn't love a wall." He never quite around to naming exactly what that something is in "Mending Wall," but I can hazard a guess of my own.
My first guess is the Holy Spirit.
God doesn't seem to be a fan of walls, literally or metaphorically. While He may have aided Nehemiah in rebuilding the walls around Jerusalem, it's fairly evident it was an exception to the rule in order to preserve the covenant community. The rest of the Bible tells a story of joining together and erasing walls. Revelation's image of worship in heaven shows us a group "from every nation, tribe, people, and language, standing before the throne" (7:9). Paul writes in Galatians 3:28, "There is neither Jew nor Gentile, neither slave nor free, nor is there male and female, for you are all one in Christ Jesus."
In Jesus, no divisions exist. It doesn't matter if you're white or Hispanic, a billionaire or a pauper. Salvation is open to everyone equally, and the Church should make no distinctions where God doesn't. The Gospel of Jesus Christ is a gospel of equality to all peoples of all groups. We as Christians should be among the loudest voices proclaiming hospitality and care for the Other, just as God commands in both Testaments.
All the lines on the map were drawn by humans, artificial barriers and walls constructed by human beings. God simply gave us a world to share and enjoy as brothers and sisters. "Something there is that doesn't love a wall," and that Something is God.