For one thing, you cannot say “came back from the death”: death takes no article here. Death works as an abstract condition not a particular instance of one, much like life or hope or joy or sadness or despair. You would not lose the hope; you would just lose hope in general. You would not return from the sadness — unless it were the sadness that befell you upon learning the hour of your death and subsequent loss of hope leading to despair. It doesn't normally get to be a particular instance of a death, let alone of the death. English uses the zero article in many places, and that part is much too big a topic for this question.
But for the main thing, here dead is a noun, usually a plural one.
Nor canst thou show the dead are dead.
Note please the plural concord with the verb are.
But the phrase “from the dead” is special. It arose from translating the New Testament. The OED says of it:
a. singular. One who is dead, a dead person. Formerly with a, and with possessive dead's (dedes, dedis).
b. plural the dead.
c. from the dead [originally translating Latin a mortuis, Greek ἐκ νεκρῶν, ἀπὸ τῶν νεκρῶν in the New Testament.] : from among those that are dead; hence nearly = from death.
Even Old English did this. The Lindisfarne Gospels written back around 950 had John 2:22 begin with this is Old English:
Miððy uutudlice ariseð from deadum,
Which in the Early Modern English of the KJV ran:
When therefore he was risen from the dead,
And in the Vulgate ran like this in Latin:
Cum ergo resurrexisset a mortuis,
So this phrase “from the dead” has been used that way ever since. It’s been in English since before you could even recognize English as English: “ariseth from deadum” looks almost silly to us these days.
While adjectives can be nominalized and used as a singular to mean the part with that property (like in the dead of night) or in the plural meaning people who have that property (like in the good, the bad, and the ugly; or judging the quick and the dead; or saying that the poor will be with you always), when you see something that looks like it’s acting like a noun, it probably is as good as “a real one” for nominal purposes like these.