I go by Jesus fulfilling the OT in the NT being the only proof I need that the NT is validated and considered Scripture and profitable for teaching. Jesus is God's word. I can't go outside of that without damning myself. I don't need to find a 'fact' or a 'specific sentence or book title' as a reason to believe.
Scripture most certainly is profitable for teaching. But the question is - how do you know what Scripture is?
You claim not to go outside of Scripture as the source for your beliefs. If that is true, then your belief that the New Testament contains 27 books should be found within the Bible itself. So then, tell me what book of the Bible states that there are 27 books of the New Testament that are the inspired word of God, and tells you what these 27 books are. You cannot, and this proves that the source of your belief is from outside of Scripture.
What is the specific basis for your belief that Hebrews, James, 1 Peter, 2 Peter, 1 John, 2 John, 3 John, Jude and Revelation are the inspired word of God, and that 1 Clement and the Didache are not? If you bought a Bible online and 1 Clement was included between Hebrews and James, would you then believe that 1 Clement is the inspired word of God? Why not?
Do you think that the Bible just dropped out of of the sky as a leather-bound volume with a blue ribbon around it that said “The books herein are the inspired word of God?”
Did the Lord appear to you in a dream and tell you that the New Testament has 27 books?
Most likely your parents told you that the New Testament has 27 books. Or your pastor told you. Or a friend who introduced you to the Christian faith told you. Or you were told by the printing company that packaged the 27 books together and printed them for you, before you purchased it at a bookstore or online.
Could it be that a group of men got together 1,650 years ago and told the members of the Church which writings they possessed were the inspired word of God, and which were not? Could it be that you accept this tradition even though you do not realize it, because you have never researched the question of how the canon of the New Testament was formed?