Here is a quote from Martin Luther regarding adiaphora and the need for uniformity of external worship practices. It comes from a letter written in 1525 entitled “A Christian Exhortation to the Livonians Concerning Public Worship and Concord.”
I have heard from reliable witnesses that faction and disunion have arisen among you, because some of your preachers do not teach and act in accord, but each follows his own sense and judgment.
This causes confusion among the people. It prompts both the complaint, “No one knows what he should believe or with whom he should side,” and the common demand for uniformity in doctrine and practice. In times gone by, councils were held for this purpose and all sorts of rulings and canons made in order to hold all the people to a common order. But in the end these rulings and canons became snares for the soul and pitfalls for the faith. So there is great danger on either side. And we need good spiritual teachers who will know how to lead the people with wisdom and discretion.
For those who devise and ordain universal customs and orders get so wrapped up in them that they make them into dictatorial laws opposed to the freedom of faith. But those who ordain and establish nothing succeed only in creating as many factions as there are heads, to the detriment of that Christian harmony and unity of which St. Paul and St. Peter so frequently write.
[Satan will] use external divisions about ceremonies to slip in and cause internal divisions in the faith. This is his method, which we know well enough from so many heresies.
Now even though external rites and orders—such as masses, singing, reading, baptizing—add nothing to salvation, yet it is un-Christian to quarrel over such things and thereby to confuse the common people. We should consider the edification of the lay folk more important than our own ideas and opinions. Therefore, I pray all of you, my dear sirs, let each one surrender his own opinions and get together in a friendly way and come to a common decision about these external matters, so that there will be one uniform practice throughout your district instead of disorder—one thing being done here and another there—lest the common people get confused and discouraged.
For even though from the viewpoint of faith, the external orders are free and can without scruples be changed by anyone at any time, yet from the viewpoint of love, you are not free to use this liberty, but bound to consider the edification of the common people, as St. Paul says, 1 Corinthians 14 [:40], “All things should be done to edify,” and 1 Corinthians 6 [:12], “All things are lawful for me, but not all things are helpful,” and 1 Corinthians 8 [:1], “Knowledge puffs up, but love builds up.” Think also of what he says there about those who have a knowledge of faith and of freedom, but who do not know how to use it; for they use it not for the edification of the people but for their own vainglory.
Now when your people are confused and offended by your lack of uniform order, you cannot plead, “Externals are free. Here in my own place I am going to do as I please.” But you are bound to consider the effect of your attitude on others. By faith be free in your conscience toward God, but by love be bound to serve your neighbor’s edification, as also St. Paul says, Romans 14 [15:2], “Let each of us please his neighbor for his good, to edify him.” For we should not please ourselves, since Christ also pleased not himself, but us all.
But at the same time a preacher must watch and diligently instruct the people lest they take such uniform practices as divinely appointed and absolutely binding laws. He must explain that this is done for their own good so that the unity of Christian people may also find expression in externals which in themselves are irrelevant. Since the ceremonies or rites are not needed for the conscience or for salvation and yet are useful and necessary to govern the people externally, one must not enforce or have them accepted for any other reason except to maintain peace and unity between men. For between God and men it is faith that procures peace and unity.
This I said to the preachers so that they may consider love and their obligation toward the people, dealing with the people not in faith’s freedom but in love’s submission and service, preserving the freedom of faith before God. Therefore, when you hold mass, sing and read uniformly, according to a common order—the same in one place as in another—because you see that the people want and need it and you wish to edify rather than confuse them. For you are there for their edification, as St. Paul says, “We have received authority not to destroy but to build up” [2 Cor. 10:8]. If for yourselves you have no need of such uniformity, thank God. But the people need it. And what are you but servants of the people, as St. Paul says, 2 Corinthians 2 [1:24], “We are not lords over your faith, but rather your servants for the sake of Jesus Christ.”
– Translated by Paul Zeller Strodach; Revised by Ulrich S. Leupold; Luther’s works, vol. 53: Liturgy and Hymns (J. J. Pelikan, H. C. Oswald & H. T. Lehmann, Ed). Philadelphia: Fortress Press. Emphasis added.