I would answer Apostolic tradition.
I stump many protestants when I ask them “Please prove to me that the Twelve Apostles held to your interpretation of scripture”
“but my interpretation is in the BIble”
“Good, show it to me.”
They will show you a Bible verse and it doesn’t say what their interpretation is, OR they show you a Bible verse and interpret it for you.
Of course, many confuse their interpretation of scripture (their tradition) with scripture. You can see right through that easily.
A good example: “Matthew 1:25 says that Joseph knew Mary not until she gave birth. Then afterwards, things changed”
Hold on. Where does it say anything about “afterwards” in that verse? Sounds like you’re adding to the scriptures, for that verse says nothing about afterwards. If it does, please show me where it says so in that verse.
They won’t find that verse.
So they’re taking their interpretation of Matthew 1:25 and making it equivalent to Matthew 1:25, when in reality it is not.