5 Surely you recall that I used to tell you these things while I was still with you.
Paul is reminding the church in Thessalonica of the teachings that he taught them while with them. He was reassuring them that they were not left behind because…
1. After this I heard something like the loud voice of a vast multitude in heaven, saying, Hallelujah! Salvation, glory, and power belong to our God,
A vast multitude is most likely the angelic beings in heaven who are witnessing the destruction of the false religion/Babylon