Adiaphora are not to deny Christian freedom, replace true worship, or interfere with the preservation of the pure doctrine...

The article about Christian freedom is at stake here. The Holy Spirit, through the holy apostle’s mouth, sincerely told His Church to preserve this article, as we have just heard. As soon as Christian freedom is weakened and human traditions are forced on the Church with coercion, as though it were wrong and a sin to omit them, the way is already prepared for idolatry. In this way, human traditions are multiplied and regarded as a divine worship, not only equal to God’s ordinances, but even placed above them.

Furthermore, idolaters are confirmed in their idolatry by such yielding and conforming in outward things, where there has not previously been Christian unity in doctrine. On the other hand, true believers are grieved, offended, and weakened in their faith. Every Christian, for the sake of his soul’s welfare and salvation, is bound to avoid both of these, as it is written:

Woe to the world for temptations to sin! (Matthew 18:7)

But whoever causes one of these little ones who believe in Me to sin, it would be better for him to have a great millstone fastened around his neck and to be drowned in the depth of the sea. (Matthew 18:6)

But what Christ says is to be especially remembered:

So everyone who acknowledges Me before men, I also will acknowledge before My Father who is in heaven. (Matthew 10:32)

However, this has always and everywhere been the faith and confession about such matters, by the chief teachers of the Augsburg Confession. We are following in their footsteps and we intend to persevere in their confession by God’s grace.

~BOC, FSD, X, 15-18

Adiaphora are not worship of God, but can be used for good order, Christian discipline, and the Church's edification...

Regarding genuine adiaphora, or matters of indifference (as explained before), we believe, teach, and confess the following:  such ceremonies, in and of themselves, are not worship of God, nor any part of it.  They must be properly distinguished from ceremonies that are.  As it is written, "in vain do they worship Me, teaching as doctrines the commandments of men" (Matthew 15:9).

We believe, teach, and confess that the community of God in every place and every time has, according to its circumstances, the good right, power, and authority to change and decrease or increase ceremonies that are truly adiaphora.  They should do this thoughtfully and without giving offense, in an orderly and appropriate way, whenever it is considered most profitable, most beneficial, and best for good order, Christian discipline, and the Church's edification.  Furthermore, we can yield and give in with a good conscience to the weak in faith in such outward adiaphora.  Paul teaches this in Romans 14 and proves it by his example.
~BOC, FSD, X, 8-9

We must not enact traditions, in addition to the Gospel, as if they are acts of forgiveness or righteousness...

We must keep in the Church the doctrine that we receive the forgiveness of sins freely for Christ's sake, through faith.  We must also keep the doctrine that human traditions are useless services and, therefore, neither sin nor righteousness should be placed in meat, drink, clothing, and like things.  Christ wished the use of such things to be left free, since He says, "It is not what goes into the mouth that defiles a person" (Matthew 15:11); and Paul says, "The kingdom of God is not a matter of eating and drinking" (Romans 14:17).  Therefore, the bishops have no right to enact traditions in addition to the Gospel, so that people must merit the forgiveness of sins, or that they think are services that God approves as righteousness. They must not burden consciences (as though it were a sin to leave such observances undone).  All this is taught by that one passage in Acts 15:9, where the apostles (Peter) say that hearts are cleansed through faith.  Then they prohibit the imposing of a yoke and show how great a danger this is, and multiply the sin of those who burden the Church.  "Why do you tempt God?" they say.  Our adversaries are not terrified even by this thunderbolt.  They defend traditions and godless opinions with violence.
~BOC, AP, XXVIII (XIV), 7-8

The errors of the kingdom of the pope detract from Christ's glory and bring destruction to souls...

In this our consciences are excused well enough, for the errors of the kingdom of the pope are clear.  Scripture with its entire voice cries out that these errors are a teach of darkness and of Antichrist.  The idolatry in the abuse of the Masses is clear.  The Masses are used for the most shameful moneymaking.  The doctrine of repentance has been utterly corrupted by the pope and his followers.  They teach that sins are forgiven because of the value of our words.  Then they tell us to doubt whether the forgiveness takes place.  They nowhere teach that sins are forgiven freely for Christ's sake, and that by this faith we obtain forgiveness of sins.

So they hide Christ's glory and rob consciences of firm consolation.  They abolish true divine services (i.e., the exercises of faith struggling with despair).

They have clouded over the doctrine about sin.  They have invented a tradition about the listing of offenses, producing many errors and despair.

In addition, they have invented satisfactions, with which they have also hidden Christ's benefit.

From these, indulgences have been borne.  These are pure lies, fabricated for the sake of making money.

How many abuses and what horrible idolatry the invocation of saints has produced!

What shameful acts have arisen from the tradition of celibacy!

What darkness the doctrine of vows has spread over the Gospel!  They pretend that vows are righteousness before God and merit the forgiveness of sins.  So they have transferred the benefit of Christ to human traditions and have completely snuffed out the doctrine about faith.  They have pretended the most silly traditions are services of God and perfection.  They have preferred these to the works of the callings that God requires and has ordained.  These errors should not be treated lightly.  They detract from Christ's glory and bring destruction to souls.  They cannot be passed by unnoticed.
~BOC, PPP, 42-48

Christian righteousness is the faith that believes that sins are freely forgiven for Christ's sake...

Not only the people, but also those teaching in the churches, have generally been persuaded to believe in making distinctions between meats, and similar human traditions.  They believe these are useful works for meriting grace and are able to make satisfaction for sins.  From this there developed that new ceremonies, new orders, new holy days, and new fastings were instituted daily.  Teachers in the Church required these works as a necessary service to merit grace.  They greatly terrified people's consciences when they left any of these things out.  Because of this viewpoint, the Church has suffered great damage.

First, the chief part of the Gospel--the doctrine of grace and of the righteousness of faith--has been obscured by this view.  The Gospel should stand out as the most prominent teaching in the Church, in order that Christ's merit may be well known and faith, which believes that sins are forgiven for Christ's sake, be exalted far above works.  Therefore, Paul also lays the greatest stress on this article, putting aside the Law and human traditions, in order to show that Christian righteousness is something other than such works.  Christian righteousness is the faith that believes that sins are freely forgiven for Christ's sake.  But this doctrine of Paul has been almost completely smothered by traditions, which have produced the opinion that we must merit grace and righteousness by making distinctions in meats and similar services.  When repentance was taught, there was no mention made of faith.  Only works of satisfaction were set forth.  And so repentance seemed to stand entirely on these works.
~BOC, AC, XXVI, 1-7

Do not bondage people to the Law with man-made traditions...

Those who establish such traditions* are acting contrary to God's command when they locate sin in foods, days, and similar things.  They burden the Church with bondage to the law, as if there needs to be something similar to the services commanded in Leviticus [chapters 1-7] in order to merit justification....It is necessary for the chief article of the Gospel be preserved, namely that we obtain grace freely by faith in Christ, and not by certain observances or acts of worship devised by people.
~BOC, AC, XXVIII, 39, 52

*ones for the purpose of making satisfaction of sins or to merit grace or righteousness (see 35)