It is important to read this quote, which comes from the final sentence of this chapter from this excellent and moving novel, in the context of the whole chapter. Sam writes about the way that the coming of the garden brought a new sense of purpose and peace and also harmony amongst the multicultural neighbourhood. However, at the same time, he then goes to chart how this sense of unity gradually becomes eroded, bit by bit, as first the lots become segregated on the basis of ethnicity, and then there were quarrels about garbage. Finally, when the crazy homeless man who slept on a broken couch had his bed taken away from him, he reacted by ripping up plants. Before you know it, people start putting fences around their gardens with big KEEP OUT signs. Sam's final paragraph says it all:

God, who made Eden, also wrecked the tower of Babel, by dividing people. From Paradise, the garden was turning back into Cleveland.

Sam is making a very sad comment about the human tendency to quarrel, to separate and to divide ourselves. Sam watches as the garden, which starts off as having so much promise and has the potential to unite a mixed group of people, then merely becomes another manifestation of man's selfishness, greed and inability to work with others. Paradise had briefly been achieved, but now it was swiftly returning to normal Cleveland.