You might think Peter and the11 knew the same thing, But that would be wrong.
"and He began to teach them that the Son of man must suffer many things, and be rejected of the elders and of the chief priests, and scribes, andbe killed, and after three days rise again. And He spake that saying openly and Peter took Him and beganto rebuke Him." (Mark8:31,32).
"For He taught His disciples, and said unto them, the Son of Man is delivered unto the hands of men, and they shall kill Him. And after that He is killed, He shall rise the third day. BUT THEY UNDERSTOOD NOT THAT SAYING and were afraid to ask." (Mark 9:31,32).
Notice, Jesus Christ did not tell them he was to die for their sins. He told them He was to die.
It was spelled out. More importantly, Jesus Christ raised up another apostle, named him Paul and gave Revelations that had never been given before. They were called the mystery. They had been hidden since the foundation of the world. Until first revealed to Paul
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.