You’re trying to tell Lutherans what they believe. It isn’t a good apologia.But you’re not a Lutheran. So, I’m not telling you what you believe. I’m telling you what Lutheran’s believe. I think some other ex-Lutherans have been telling you the same thing I’m telling you.
For a Lutheran, it means a teaching found in scripture. You’ve looked at the Book of Concord. It’s right there in the FC, Epitome, Summary Rule and Norm.Doctrine means teaching. You have to teach people about SS. It isn’t something that anyone picks up by osmosis and begins to do.
We believe, teach, and confess that the sole rule and standard according to which all dogmas together with [all] teachers should be estimated and judged are the prophetic and apostolic Scriptures of the Old and of the New Testament alone, as it is written Ps. 119:105: Thy Word is a lamp unto my feet and a light unto my path. And St. Paul: Though an angel from heaven preach any other gospel unto you, let him be accursed, Gal. 1:8.
It is pretty clear what they mean