I can understand your confusion. Here is how the sentence should be parsed: (most of us) (will fall) (long before) (we see). So, the expression is not "fall long." It's saying that a long time before we see the grand gates, most of us will have already fallen. Hope that helps.

"fall long" isn't really an expression, it's the end of one expression and the beginning of another. "... most of us will fall" describes what will happen (in this case, they will fall or fail to continue the journey) "long before we see..." describes when it will happen (a long time before the final destination is reached)

In this context, “fall long” means to fail or give up before reaching the goal. The speaker is using poetic language. “Fall” means to stumble, fail, or stop trying. “Long before” means much earlier than expected. So the sentence “most of us will fall long before we see the grand gates” means most people will not reach their final destination or goal; they will give up or fail along the way.

fall long before = fall a long way short/prior = fall down, far before Without seeing even more context, I'm guessing it means either we will die before reaching the gates of some destination, or we will fail and go to hell before reaching heaven's gates.