I have to disagree. They were waiting for the King to set up his Kingdom on earth so they can go to the ends of the earth preaching the KINGDOM gospel.
They thought they would go through the trib, survive to the end and become priests to the gentiles.
But God opened up the timeline and that’s the mystery that Paul speaks of. : )
Gibberish!
The only ‘mystery’ that Paul presented was the Savior’s death releasing his bride to remarry (Romans 7)
None of the apostles were so deluded as to be waiting for the kingdom; they saw the kingdom partly come to them at Pentecost.
They wrote of how the fullness of the kingdom would come at the last trump.
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