The truth in progressivism
We can find a core of truth in any "successful" ideology, without which it would be too implausible to gain any traction. For progressivism, that core is the dramatic advances we have made in science and engineering over the last four centuries. They give credence to the idea that we are "more advanced" than Shakespeare or Dante, that being "modern" is better than being "Medieval," and that next year we will be even more advanced than this year. Sin, in progressivism, is being on "the wrong side of history," since tomorrow is always better than today. (Except, of course, when it isn't, which is when the "reactionaries" have temporarily reversed the march of progress.)
But consider running into a friend you have not seen for a year. He was a bit pudgy the last time you saw him, but now he is rippling with muscles.
"What happened to you?" you ask him.
"I've been going to the gym: I am a lot more advanced than when you last saw me, aren't I?"
"Well, you certainly are more muscular. But what do you mean by 'advanced'?"
"I mean that I am more modern than you. For instance, look how many push-ups I can do!" (He drops and does fifty.)
"Ah, so you have advanced beyond me in fitness?"
"No, no, in every way: ethically, philosophically, artistically... I am progressive."
"But where is your evidence that, for instance, you are ethically more advanced than me?"
"Look at how long I can hold a leg lift!"
Your friend is of course talking nonsense: simply because he is made progress at fitness does not say anything about whether he has made progress in any other area. Similarly, the fact that our science is more advanced than the Elizabethans has no relevance for judging whether our theater is better than theirs; the fact that we can build bridges better than the ancient Greeks says nothing about whether we can philosophize better than them; the fact that Buddha did not have a smartphone does not mean that we have a better grasp of the meaning of life than he did.