Let me see if I can help you in some small way.
Okay, St. Paul was a persecutor of the Church: hated Christians, persecuted saints, watched Stephen die, killed.
His was a unique conversion, but shows what kind of people can be converted, viz. the meanest, most cruel, hateful people (think about that next time politicians are discussing enemy leaders).
Paul was converted by seeing the glory of God.
That alone is enough in my eyes to validate showing what sort of things he said after he was converted.
I know though that that doesn't answer the basis of your question though.
There are two answers to the question of who canonized Paul.
1. It was God.
God had a lot to say about salvation and divine Grace and that sort of thing that would have sounded best coming from the mouth of a converted murderer and Church-persecutor.
2. The Council of Nicea.
There were canon lists being made up already before Nicea, but here is where the canon was officialy set.
I believe that God had his hand in this too though.
So that is the mechanism, but what about the why?
On the both the divine and human levels, it would make sense that, even though prettier and more euphonic Christian writings have come out later they were written mostly on the basis of what was taught through Paul, the Gospels, Peter, John, James, etc.
So it could be said that Paul's books were included to act as precedent to show what is important to the Faith.
Romans is there as the basis for interpreting everything as Grace.
The Corinthian letters reveal how important Church unity is.
Titus, to show that even though Paul talks about Grace he does not think of works as being as bad as some modern, [lazy
] interpreters say.
In a sort of circular manner though, I suppose that the end of the matter lies in that the Bible shows what Christians believe and Christians believe what the Bible teaches.
Forgive me for any fuzziness or unclarity, I just woke up.
But if you'd like me to elaborate on some idea that I might not have developed enough, just feel free to ask.
