And a word that shall echo forevermore! (line 124)
This is the image that connects the past, present, and future. Paul's warning continues down through history. We're supposed to feel like our history is alive, that if we tried hard enough, we could actually hear that shout. Notice that Longfellow doesn't tell us what this "word" is. In fact, we never get a direct quote about what Paul said as he rode through the villages.
For, borne on the night-wind of the Past, Through all our history, to the last (lines 125-126)
Now he really drives it home. In case you didn't get it before, the message of Paul Revere is everlasting and will continue to be heard even after America stops existing. Of course, Paul is getting a little help from the poem. Shmoop wonders how many people would care about Paul Revere if this poem hadn't been written. Check out how careful the speaker is to identify his listeners. This is "our" history. Other people can listen in, but this seems to be a message aimed specifically at Americans.