In "At Home in the House of my Fathers", p. 119 and onwards there is an essay by C.F.W. Walther delivered in 1858. Previously a portion was translated by Guebert, now the remainder by Harrison.
In his introductory comments the latter says:
"The only guarantee we have in remaining truly Lutheran is in our church--including its pastors and church workers--confessing genuine Lutheran doctrine. That doctrine is presented in the Book of Concord. Walther's lucid and pointed defense of full, uncompromised subscription to the Lutheran Confessions is a clarion call for us in times of division and challenge to rally around what we share in our great affirmation of the Lutheran Symbols... It is the very content of this essay which has allowed the Missouri synod to endure to this day as a confessional body unlike the Lutheran churches against which he writes, which are now all part of the ELCA. This essay is all the more amazing in that it was prepared by Walther on the precipice of his breakdown. Let us be charitable, but let us fully confess the Book of Concord because it is in full accord with the Holy Scriptures!"
From here on we have Walther.
"The Symbols are confessions of faith or of the doctrine of the church and never were intended to be anything more nor less; therefore, an unconditional subscription to the Symbols can be interpreted in only one way.
An unconditional subscription is the solemn declaration that the individual who wants to serve the Church makes under oath 1) that he accepts the doctrinal content of our Symbolical Books because he recognizes the fact that they are in full agreement with Scripture and do not militate against Scripture in any point, whether that point be of major or minor importance; and 2) that he therefore heartily believes in this divine truth and is determined to preach this doctrine without adulteration."
There is quite a bit more detail elucidating what this means exactly. I will skip that for now. I should probably go back and study that more. He finishes this section with the contrary:
"Here it is self-evident what it means on the other hand to subscribe only conditionally to the Symbols"
By a conditional subscription to the Symbols, the subscriber does not pledge himself to accept every doctrine contained in the Symbols as in full agreement with Scripture and reserves the rightly to distinguish between the doctrines presented. In the course of time, various formulations of a conditional subscription have been advocated.
1. A man may subscribe to the Symbolical Books "if" and "insofar as" [quatenus] they do not militate against Scripture or "if" and "insofar as" they agree with Scripture. The so-called Pietists employed this conditional formula, and later on the Rationalists. However, it should be stated that by using this formula, the Pietists did not want to yield they fundamental articles of our faith. The Rationalists, on the other hand, did not want to be bound to these articles, even as they accepted Scripture as a rule and norm for their teaching only insofar as the content of Scripture was not contrary to their reason.
2. A man subscribes conditionally if he accepts the Symbols insofar as he believes that they teach the fundamental doctrines of the Bible correctly or teach them in a matter substantially correct. [This is the manner in which the present-day General Synod and its individual synods confess the Augsburg Confession.]
3. Some want to subscribe to the Symbols with the proviso that they may interpret them according to Scripture or understand them correctly. This was the condition under which the Reformed declared themselves ready to subscribe to the Unaltered Augsburg Confession. ... Zanchi subscribed to the Augsburg confession in 1563 with the following words: 'I accept the form of doctrine to the extent that I acknowledge it as godly.'... To the extent that I recognize and regard it for godly, I accept it as such, so long as it is understood as I have explained it. The Calvinist Peter Martyr wrote to the Landgrave of Hesse in 1565: 'I am happy to accept the Augsburg Confession, if it is properly and suitably understood.'
This reminds me of what my grandmother used to say: "Ich esse alles gerne--was ich selber koche." Which is: "I like to eat everything--that I've cooked myself."
In a similar manner, some few years ago, an entire Lutheran pastoral conference gathered in Fuerth in Bavaria, led by Herr Pastor Loehe, expected our Synod to understand and interpret the Symbols according to the Scriptures, in order to come to unity in light of the doctrines of Church and Office that had become controverted. The conference stated, 'We do not doubt--provided we accept the Word of God as the sole rule in all matters--that we understand our good confessions according to the precedent of the Scriptures, and in light of the opposition fought against at that time, then the Spirit of Truth will also lead his church into all truth.' With similar assertions they intended to subscribe to the Confessions, if one should dare to understand them correctly. But this cannot mane merely daring to understanding the Confessions as they actually sound and are intended. For only someone who is insane can understand them differently. Such statements merely indicate that the Confessions cannot be accepted as they read and as they were actually intended. In so doing one reserves the right to put a meaning into the words of the Confessions which is not actually present, but which a person holds to be correct and in accord with he Bible.
It is exactly the same kind of matter when it is asserted that the Symbols are to be accepted in their 'historical context'...
4. Another conditional subscription is to subscribe only to that which is confessional in the Symbols. Pastor Loehe, for instance, subscribes to the symbols only with this condition. He writes: 'I differentiate in the Book of Concord between what is said by way of confession and what is not said as confession. And I distinguish still more. I am disinclined to hold on to the letter and allow myself to be guilty of symbolatry....' it is self-evident that such a conditional subscription excludes a considerable portion of the doctrinal content of the symbols from that which on can confess as his faith [as then Pastor Loehe bluntly declares in the same document the idea], that other parts of the doctrine which are expressed in the Symbols are not pure and therefore are subject to clarification.
5. A fifth type of conditional subscription is one that demands the right to subscribe to the Symbols of both he Lutheran church and of the Reformed church if and insofar as they agree with each other. The Union Church, in which this form of oath is used, as is well know, not only excludes several of the chief doctrines in the Symbols as nonbinding but also leaves the question undecided as to which doctrines these are.
6. Others have subscribed with the reservation to regard as open questions even those points a controversy has arisen, e.g., the question concerning the church and the Ministry. They assert that these must be viewed an treated as open questions...
7. Finally, the seventh and coarsest manner of a merely conditional subscription to the Church's Confessions is the manner of the Rationalists. They do bot pledge themselves to the letter, but to the so-called spirit of the Symbolical Books. it is evident that a mere conditional subscription runs counter to the purpose of the symbols in general as well as to the purpose of the pledge in particular.
So much for today.
Just the transition to the remainder of the essay:
"Now we proceed toward answering our question: Why are the Symbolical Books of our Church to be subscribed to by ministers of the same not conditionally, but unconditionally? Answer: Because a merely conditional subscription contradicts both the purpose of the Symbols in general and also the purpose of the oath of subscription in particular."