In that speech, he alludes to the prayer "on earth as it is in heaven" and connects the work here below, with the work above. He talks about our responsibility to not just reflect on the world to come but the implications of our work in this world. The long white robes over yonder is an allusion to angels in heaven. The new Jerusalem is an allusion to the end times when there is a new heaven and new earth.
It's all right to talk about "long white robes over yonder," in all of its symbolism. But ultimately people want some suits and dresses and shoes to wear down here! It's all right to talk about "streets flowing with milk and honey," but God has commanded us to be concerned about the slums down here, and his children who can't eat three square meals a day. It's all right to talk about the new Jerusalem, but one day, God's preacher must talk about the new New York, the new Atlanta, the new Philadelphia, the new Los Angeles, the new Memphis, Tennessee. This is what we have to do.
What follows is the last part of that now famous speech.