When John, Mark, and Paul and the others wrote these books, they had no idea they would one day become as important to the New Testament church as the Old Testament scriptures had once been to those of the old covenant, they were simply writing down what they had witnessed and saw, under the direction of the Holy Spirit.
It wasn't until these epistles were collected and canonized that man was able to see that everything Christ had told us we needed to find salvation,.. was in these books, and along with that, how to live that Christian life.
That is why the apostles never said, "all religion will be based on these scriptures", because they didn't realize from their small part they played, that combined, it would contain everything we needed.
What has the RCC's added to their doctrine of salvation that was not obviously avaliable in the canon scripture?
They were collected and canonized? What book is that in? Can you give me a reference?
The act of canonization itself is extra-scriptural (i.e. there is no "table of contents").
So your second question answers itself: What is one thing the Church has added that is not contained in the canon of Scripture? - The canon of Scripture.
The only way they could "see" this was to compare what they had already learned from the Church and the way they were taught to live with what was written. Fact is that we normally see in Sacred Scripture what a particular teacher/tradition tells us we need to be looking for.
Interesting, JH. This makes a lot of sense.