One of the major sins in Judaism is idolatry; “thou shall not have other gods before me.” Sometimes the form of gods can be clearly seen as it was the case with the golden calves. But sometimes it took subtler forms which looked good indeed.
And I think eschatologizing a certain era in human history fits this category really well. What I mean is a kind of attitude which, really, almost worshipped accomplishments of a certain period at a certain place by a certain people. This attitude is usually followed by a longing to replicate those accomplishments now. It is as if the kingdom of God has arrived then and there, hence we need to redo it now and here. The time frame might not be in the past, though. If conservatism believed in the goodness of a certain past, then liberalism believed in the perpetual progress of humanity — the time is now, and in the future. The common thread is a belief in a golden era.
This is of course the exact opposite of what I wrote around a month ago, where contemporary relevance is the game of the day; both the mean and the goal by itself. Belief in a golden era does not try to be relevant. Indeed, it shows that actually being irrelevant is the true way to save humanity from its demise.