That is true, obviously. In Matthew 16 itself, actually, our Lord told the disciples of His soon-to-come sufferings in Jerusalem, and Peter declared that it would never happen to Him (Matt. 16:21, 22). Peter even REBUKED the Lord for it!
And in subsequent announcements of the same, they did not understand and it is said even to be “hidden” from them (Luke 18:33, 34).
The Twelve did not understand the work of the Cross and Resurrection until AFTER the Resurrection.
If the Gospel we preach and by which we are saved now of necessity includes the Death, Burial, and Resurrection of Christ (1 Cor. 15:1-4), then the Gospel that the Twelve and the Seventy were commissioned to preach during Christ's earthly ministry (Matthew 10; Luke 9, 10) had to be a different message, because they did not understand the sufferings of Christ.
Further, Paul was given advanced revelation from the Head of the Church seated in the heavenly places after the Resurrection and detailed after Acts ch. 28, that Peter himself had a hard time understanding (see 2 Peter 3:15, 16).
LOL! That's a good example.
If the Gospel we preach and by which we are saved now of necessity includes the Death, Burial, and Resurrection of Christ (1 Cor. 15:1-4), then the Gospel that the Twelve and the Seventy were commissioned to preach during Christ's earthly ministry (Matthew 10; Luke 9, 10) had to be a different message, because they did not understand the sufferings of Christ.
Yes, that makes sense. However, while that message may not have been as developed as the one we have today, I would doubt that it contained error. (I know you didn't say that.)