In Revelation 7:4 who is the 144,000 and what is the significance of this verse?

I believe that the true seed of Abraham is made up of both ethnic Israel and engrafted Gentiles who have been elected from the foundations of the earth. However I am trying to get a clear understanding of how that fits within the context of this chapter.
Most Dispensationalists say that it is referring to 144,000 Jews from the 12 tribes of Israel that are left after the rapture to evangelize the world. However, I think that is a highly unlikely interpretation because (among other reasons) it wasn’t held before Darby popularized the view in the 1800s.

I would also like to see what commentators such as William Hendriksen have to say about the matter. Unfortunately I don’t have access to his commentary.

Tom