Q. Does Isaiah 26:19 also support the resurrection of the Old Testament saints at Christ’s second coming or is this passage speaking about the nation of Israel’s restoration/salvation?
A. Let’s take Isaiah 26:19-21 as a single thought to get the full intent of the passage.
But your dead will live, Lord; their bodies will rise. You who dwell in the dust, wake up and shout for joy. Your dew is like the dew of the morning; the earth will give birth to her dead.
Go, my people, enter your rooms and shut the doors behind you; hide yourselves for a little while until his wrath has passed by. See, the Lord is coming out of his dwelling to punish the people of the earth for their sins.
This passage has to be a prophecy about the rapture of Church because it shows both the dead being resurrected and the living being hidden away before the time of God’s wrath begins. Old Testament saints will not be resurrected until after the Great Tribulation (Daniel 12:1-2) and likewise for the restoration of Israel.
I believe when Paul used the phrase “according to the Lord’s own word” when introducing the rapture in 1 Thes. 4:15-17 he was referring to this passage. There is nothing like it in the Gospels.