Well, technically, the content of the NT (at least the four main Gospels) are not really the teachings of the Apostles. It’s more accurate, I think, according to Church teaching, to say that the Apostles were the writers of the Gospels, but they were not the authors. God is, or was, rather, the author. And while the Church has always found inspiration in the Gospels, Church teaching is also based on tradition (the verbal handing down of teaching).
For instance, you won’t find anywhere in Sacred Scripture (NT) where it says that the Blessed Virgin Mary (who was, and still is, the very best of Jewish mothers!) was assumed into Heaven. But according to tradition, the Church has always believed that she was assumed into Heaven. After she died, her body disappeared, and lilies were suddenly found growing outside of her home, where she had died. This is just one example; there are many others.