Before it is possible to determine what is to be regarded as Christian doctrine, it is necessary to agree on the source from which such doctrine is to be drawn, and on the norm by which it must be judged; otherwise it is impossible to reach an agreement.
No one can tell us what God wants us to believe and to do but God Himself. (1 Cor. 2:9-11). Therefore our knowledge of God and of His will toward us can be derived from no other source than from God's own Word. (Is. 8:19-20) This Word of God must also be the norm and criterion according to which teachings and teachers are to be judged. (John 8: 31-32) and (1 Peter 4:11)
The Lutheran Confessions, therefore, state: 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. (Triglot, p. 777)
My thoughts: It is no wonder that confessional Lutheran churches simply cannot join with most other churches in matters of faith. It's not that we don't want to; we cannot out of adherence to scripture.