Francis Pieper wrote this about confessional subscription in the Lutheran Church Missouri Synod (LCMS) of his day:
This same truth — that the Lutheran Church does not set up in its Symbols a second norm alongside of Scripture — is evidenced by its insistence on the quia form of subscription. It binds its teachers to the doctrine contained in the Confessions not because it is the doctrine of the Confessions, but because it is the doctrine of Scripture. (Christian Dogmatics, Vol. 1, 354)
You’ll recall that quia is the Latin word for “because.” We subscribe the Confessions because they are biblical, not insofar as (quatenus) they are biblical.
From this beautiful volume entitled Concordia: the Lutheran Confessions, we find that the LCMS still apparently holds to this position:
Needless to say, confessional subscription in the nature of the case is binding and unconditional. A subscription with qualifications or reservations is a contradiction in terms and dishonest. (Concordia, xxix)
Now what strikes me is the sheer volume of the Lutheran confessional writings. Concordia is a big book! Lutheran pastors subscribe a lot more content than Reformed pastors do.
I raise that because sometimes it’s said (and I’ve said it too) that Presbyterians have to take a different approach to subscription of their confessions because they’re much more bulky and detailed. You can’t expect a Presbyterian to hold to every single detail of the Westminster Standards. So, we find things like “good faith” and “system” subscription. I find it interesting that Concordia is probably ten times bigger than the Westminster Standards and yet the LCMS apparently holds to full, quia subscription.