a great sound of a trumpet. Greek "a trumpet and a great sound" = a trumpet, yea, a great sounding trumpet. Figure of speech Hendiadys (App-6); not two things, but one.
they shall gather, &c. Quoted from Deu_30:4. See 1Th_4:16, 1Th_4:17.
His elect. Who "received the Word". Act_2:41. 1Th_2:13.
from = out of. Greek. ek.App-104.
heaven = heavens. Plural See note on Mat_6:9, Mat_6:10.
know = get to know. Greek. ginosko.App-132. The same word as in verses: Mat_24:33, Mat_24:39, Mat_24:43 ("know").