Salvation always comes through judgment in various ways. Salvation for the nation of
Israel at the Exodus came through the judgment of Egypt, and this pattern is repeated
throughout the OT. When God saves Israel, he delivers the nation by bringing judgment
on the nation’s enemies.
Salvation for all believers of all ages is made possible by the judgment that falls on Jesus
at the cross. The cross allows God to be just and the justifier of the one who has faith in
Jesus (Rom 3:24–26). Even though members of the remnant live before Jesus, saving
faith is explicit trust in the promises of God. I believe the promises of God begin in
Genesis 3:15, and I think that many of these promises concern an anointed redeemer
whom God will raise up to accomplish the salvation of his people—Messiah. So even
though OT saints don’t know that the Messiah will be named Jesus, grow up in
Nazareth etc., they have heard God promise to raise up a man who will save them and
they trust God to keep his word. So they are saved by faith in God’s promised Messiah.