Consider:
--outside of Rev (incl'g Rev10:7), the phrase "his [/thy/My] servants the prophets" is only used in the OT (26x OT; 4x Rev)
--when speaking of the NT ones, the phrase is usually [instead] "apostles and prophets" [in that order] (when speaking of the plurals like this ^ above)
--the phrase "the mystery of God" is used only once elsewhere in Colossians 2:2-3, where it says (and I think the nasb gives the proper sense also, see below) "of the mystery of God, which is Christ, in whom are hidden all the treasures of wisdom and of knowledge"
or, [nasb] "of God's mystery, that is, Christ Himself, in whom are hidden all the treasures of wisdom and knowledge."
All that to say, I am not inclined to believe that Rev10:7 ['mystery of God'] is speaking of "our Rapture" event (THAT "mystery" item in particular), but more along the lines of what Acts 3:21 was saying, "whom indeed it behooves heaven to receive until the times of restoration of all things of which God spoke by the mouth of His holy prophets from the age." [the OT prophets and prophecies, and esp concerning the events leading UP TO and surrounding His Second Coming to the earth, FOR the promised and prophesied EARTHLY Millennial Kingdom (Jesus spoke of it MUCH in the Gospels!)]