Mmm I don`t seem to have that scripture in my Bible about Thomas going to India.
Oh, OK, Ms. "Bible Scholar", the Apostles never went to the nations--they disobeyed Jesus Christ.
No, we don't have Thomas going to India in the Bible, but we do have Peter going to the Gentiles (which is another word for "the nations").
As to the `good news,` yes Peter and Paul agreed that Jesus was Lord and Messiah and that the Holy Spirit was given to the Gentiles, however, where do you read of Peter teaching about the Body of Christ? That was the revelation given to Paul which Peter could not understand. (2 Peter 3: 16)
1. Peter taught that we were all being built up into a spiritual house of God, so he did see that we were all one entity, but, indeed, Paul was given a revelation about the truth that we don't have a record of Peter teaching--that doesn't mean it was something that applied only to "Pauline" believers (which sort of idea Paul denounces as "carnal"--"I am of Paul"). Do you know what "revelation" means? It means something is being "revealed". That doesn't mean it isn't already true, it just means that someone is being given an understanding about a reality that already is.
2. It does not follow from the fact that Peter did not teach about the body of Christ, that, "ergo", that was what Peter was talking about when he said that some things Paul writes about are difficult to understand. How is it difficult to understand? I can think of other topics, like faith and works and justification, that men have debated about endlessly, that Peter could have been referring to. And getting that topic wrong could actually affect your salvation, as he says--"to their own destruction"--but misunderstanding a simple idea like "the body of Christ" would be a difficult feat, and it likely wouldn't result in someone's damnation.