1. As Scott Hahn points out, nowhere in the Scripture does it say, "Sola Scriptura."
2. In several places, the gospel writers testify that many other things were said that were not written down. Some of these things were passed along in the early Church without being included in the canonical scriptures. So their authority is less certain, but not simply dismissable out of hand.
3. Even what is written is unclear. Thus the great passage in Revelation has been differently interpreted by Catholics and some Protestants. The Woman in the Wilderness to Catholics is Mary, but for some early Protestants was the true Church in exile. I must say that I find that a bit of a stretch.
12:1 And there appeared a great wonder in heaven; a woman clothed with the sun, and the moon under her feet, and upon her head a crown of twelve stars:
12:2 And she being with child cried, travailing in birth, and pained to be delivered.
12:3 And there appeared another wonder in heaven; and behold a great red dragon, having seven heads and ten horns, and seven crowns upon his heads.
12:4 And his tail drew the third part of the stars of heaven, and did cast them to the earth: and the dragon stood before the woman which was ready to be delivered, for to devour her child as soon as it was born.
12:5 And she brought forth a man child, who was to rule all nations with a rod of iron: and her child was caught up unto God, and to his throne.
12:6 And the woman fled into the wilderness, where she hath a place prepared of God, that they should feed her there a thousand two hundred and threescore days.
Very hard to see how this Woman who gives birth to a man child who will rule the universe and put down Satan and be seated on the Throne of God, is not Mary, the Mother of Jesus.