He refers to the physically dead – their souls are very much alive with Christ. They have been joined to him; they are in him; they shall rise first.
You think of the effect such a thing would have on the unbelieving world, on people who have scorned and derided, on people who have trained their minds to believe there is nothing supernatural, nothing spiritual, no God, nothing but that cannot be explained by material processes. To see the vast upheaval of resurrection, beings being formed before their eyes, and the bodies of those who have gone to be with Christ being rescued from the substance of earth will be just astonishing.
We walk by faith. We are belittled, we are derided, we are set aside, but the day is coming when Christ will come, and these astonishing, really indescribable things will take place, and it will be the day of triumph and of glory for the Saviour of the world.