What did Herbert Croly mean when he said "That homogeneity [of our nation] has disappeared never to return"?
What Herbert Croly was saying here (in his book The Promise of American Life) was that America had grown too diverse in a number of ways to remain homogeneous in its attitudes and beliefs.
Croly says that early America was a society in which there was a strong push for everyone to have the same values. At that time, Americans tended to live in small communities where they could be easily pressured into acting and thinking in certain ways.
As America became more of a commercial society, Croly says, this kind of homogeneity fell apart. People became less dependent on and less connected to their neighbors. This meant that it was much less possible for pressure to be put on them to confrom. Because of this change, Americans became less closely connected to one another (which is bad), but they were no longer pressured to conform (which may be good).