Your example in Mark happened before and not after. We do not know what was revealed after He rose and after the Holy Spirit came upon them. We only know that they came to understand it at some point.
And, may I gently suggest that they did this without the New Testament writings of Paul?
Was it Paul’s oral recounting of what Jesus revealed to him that led them to know?
Would this be an example of what Jesus meant when He said that He had more to teach them but must leave so that He could send the Holy Spirit to remind them and lead them to all truth?
If it is true that the fact that Jesus died for our sins was hidden and not revealed in the OT, that it was a mystery until revealed to Paul, make the case for the declaration of dogma by the Church?
It would seem so if that HUGE nugget of truth was not expressed in the OT, since it is the Apostles’ claims about the OT(since that was the only Scripture they knew of)that you use to support sola scriptura and deny that the Church could know something that is not explicitly state in Scripture.
On the day of Pentecost Peter preaches a message of restoration for the nation Israel. Acts 2 and 3 mark the beginning of the harvest for Israel. It is the preparing of the Jewish people for the times of restitution when Jesus Christ will come to earth the second time. Peter never mentions Christ dying for our sins.
Isaiah 53 is a prophecy of the Messiah. I believe it very clearly states the role he would have in our salvation. It is not the only place in the Old Testament where Christ’s sacrifice for our sins was told. An example is Isaiah 53:6
6 We all, like sheep, have gone astray,
each of us has turned to his own way;
and the LORD has laid on him
the iniquity of us all.