A.T. Robertson Commentary Revelation 10:7

"but in the days of the voice of the seventh angel, when he is about to sound, then is finished they mystery of God, according to the good tidings which he declared to his servants the prophets." — Revelation 10:7 (ASV)

When he is about to sound (οταν μελλη σαλπιζειν). Indefinite temporal clause with οταν and the present active subjunctive of μελλω and the present (inchoative) active infinitive of σαλπιζω, "whenever he is about to begin to sound" (in contrast to the aorist in 11:15).

Then (κα). So in apodosis often (14:10).

Is finished (ετελεσθη). First aorist passive indicative of τελεω, proleptic or futuristic use of the aorist as in 1 Corinthians 7:28. So also 15:1.

The mystery of God (το μυστηριον του θεου). This same phrase by Paul in 1 Corinthians 2:1; Colossians 2:2. Here apparently the whole purpose of God in human history is meant.

According to the good tidings which he declared (ως ευηγγελισεν). "As he gospelized to," first aorist active indicative of ευαγγελιζω, a rare use of the active as in 14:6 with the accusative. See the middle so used in Ga 1:9; 1 Peter 1:12. See Am 3:7; Jeremiah 7:25; 25:4 for this idea in the O.T. prophets who hoped for a cleaning up of all mysteries in the last days.