I think I just found out what might have led Joseph Mede to conclude that the First Resurrection would only be for the martyrs. He referenced Tertullian in his letter. I've been unable to find anything Tertullian said that was relevant, but I did find this quote from Commodianus that may have influenced that line of thought.
Commodianus (200-250 AD) said this,
"From heaven will descend the city in the first resurrection; this is what we may tell of such a celestial fabric. We shall arise again to Him, who have been devoted to Him. And they shall be incorruptible, even already living without death. And neither will there be any grief nor any groaning in that city. They shall come also who overcame cruel martyrdom under Antichrist, and they themselves live for the whole time, and receive blessings because they have suffered evil things; and they themselves marrying, beget for a thousand years." (Commodianus, chapter XLIV, On the First Resurrection)
Perhaps he thought that because their life was cut short by martyrdom, they would get a chance to pick back up where their life was cut short the first time. That is, return to a mortal life at the same age and state at which their were murdered the first time. Of course, the problem with that theory is that it means they would die a second time. It also begs the question as to why only martyrs would be treated this way. Why not those that died early of disease or in accidents or war?
There are others who argued things that would contradict this. Origen said,
"it will appear vain and superfluous for any one to arise from the dead in order to die a second time" (Origen, circa 230 AD, in De Principiis, Book 2, Chapter 10)
So it would seem Origen did not share this view. In fact, I am not sure I know of a second person who agrees with Commodianus on this topic. But perhaps Mede was influenced by Commodianus' logic.