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 that is compelled or commanded obscures pure doctrine and true worship...

We also believe, teach, and confess that at a time of confession, when the enemies of God's Word want to suppress the pure doctrine of the Holy Gospel, God's entire church, indeed, every single Christian, but especially the ministers of the Word, as the directors of the community of God [Latin:  God's church], is bound by God's Word to confess the doctrine freely and openly.  They are bound to confess every aspect of pure religion, not only in word, but also in works and actions.  In this case, even in adiaphora, they must not yield to the adversaries or permit these adiaphora to be forced on them by their enemies, whether by violence or cunning, to the detriment of the true worship of God and the introduction and sanctions of idolatry.  For it is written:

For freedom Christ has set us free; stand firm therefore, and do not submit again to a yoke of slavery. (Galatians 5:1)

Yet because of false brothers secretly brought in—who slipped in to spy out our freedom that we have in Christ Jesus, so that they might bring us into slavery—to them we did not yield in submission even for a moment, so that the truth of the Gospel might be preserved for you. (Galatians 2:4-5)

In this place Paul speaks about circumcision, which at that time had become an adiaphoron.  At other times circumcision was observed by Paul.  The false apostles pushed circumcision in order to establish their false doctrine that the works of the Law were necessary for righteousness and salvation.  They missed circumcision to confirm their error in people's minds.  Therefore, Paul says that he would not yield even for an hour, in order that the truth of the Gospel might continue unimpaired.

Paul yields and gives way to the weak concerning food and the observance of times or days.  But to the false apostles, who wanted to impose these on the conscience as necessary things, he will not yield even in matters that are adiaphora.  "Therefore let no one pass judgement on you in questions of food and drink, or with regard to a festival or a new moon or a Sabbath" (Colossians 2:16).  When Peter and Barnabas yielded somewhat in such an emergency, Paul openly rebuske them according to the truth of the Gospel as people who were not acting right in this matter.

This case is no longer a question about outward matters of indifference, which in their nature and essence are free. They cannot produce a command or prohibition that they must or must not be used.  The case presented by Paul is a question, in the first place, about the outstanding article of our Christian faith.  For the apostle testifies, "so that the truth of the gospel might be preserved for you" (Galatians 2:5).  The Gospel is obscured and perverted by such compulsion or command, because such adiaphora are publicly required for the approval of false doctrine, superstition, and idolatry, and for the suppression of pure doctrine and Christian liberty, or at least are abused for this purpose by the adversaries are and viewed this way.
~BOC, FSD, X, 10-14

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