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

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

Rest in the mystery of Christ's dual nature that you might have the constant consolation of truth rather than the doubt of crafty reasoning...

Since in the Holy Scriptures Christ is called a mystery upon which all heretics dash their heads, we admonish all Christians not to arrogantly indulge their reason in crafty investigations about such mysteries.  With the beloved apostles, they should simply believe.  They should close the eyes of their reason and bring their understanding into captivity to the obedience of Christ, and rejoice without ceasing in the fact that our flesh and blood is placed so high at the right hand of God's majesty and almighty power.  In this way we will certainly find constant consolation in every difficulty and remain well guarded against deadly error.
~BOC, FSD, VIII, 96

Christ, in both natures, is with us in all our troubles...

When such majesty is denied to Christ according to His humanity, we regard it as a deadly error.  For by this the very great consolation mentioned above is taken from Christians, which they have in the promise about the presence and dwelling with them of their Head, King, and High Priest.  He has promised them that not only His mere divinity would be with them (which to us poor sinners is like a consuming fire on dry stubble).  But Christ promised that He—He, the man who has spoken with them, who has experienced all tribulations in His received human nature, and who can therefore have sympathy with us, as with men and His brethren—He will be with us in all our troubles also according to the nature by which He is our brother and we are flesh of His flesh.
~BOC, FSD, VIII, 87

The assurance and confirmation found in the Lord's Supper...

The divine and the human nature have this personal union with each other in the person of Christ and have the communion resulting from it (in deed and truth).  For this reason, there is attributed to Christ (according to the flesh) what His flesh, according to its nature and essence, cannot be by itself.  Apart from this union, His flesh cannot have these attributes:  His flesh is a truly live-giving food and His blood a truly life-giving drink.  The two hundred Fathers of the Council of Ephesus have testified that Christ's flesh is a life-giving flesh.  Therefore, this man only, and no man besides, either in heaven or on earth, can say with truth, "Where two or three are gathered in My name, there am I among them" (Matthew 18:20).  Also, "And behold, I am with you always, to the end of the age" (Matthew 28:20).

We do not understand these testimonies to mean that only Christ's divinity is present with us in the Christian Church and Congregation, and that such presence does not apply to Christ according to His humanity in no way whatever.  For in that way Peter, Paul, and all the saints in heaven—since divinity, which is everywhere present, dwells in them—would also be with us on earth.  However, the Holy Scriptures say this only about Christ, and no other man.  We hold that by these words the majesty of the man Christ is declared.  Christ as receive this majesty, according to His humanity, at the right hand of God's majesty and power.  So also, according to His receive human nature and with the same, He can be, and also is, present where He wants to be.  He is present especially in His Church and congregation on earth as Mediator, Head, King, and High Priest.  This presence is not a part, or only one half of Him.  Christ's entire person is present, to which both natures along, the divine and the human—not only according to His divinity, but also according to, and with, His received human nature.  He is our Brother, and we are flesh of His flesh and bone of His bone.  He has instituted His Holy Supper for the certain assurance and confirmation of this, so that He will be with us, and dwell, work, and be effective in us also according to that nature from which He has flesh and blood.
~BOC, FSD, VIII, 76-79

Christ fulfills His office in, according do, with, and through both natures...

In fulfilling Christ's office, the person does not act and work in, with, through, or according to only one nature.  It works in, according to, with, and through both natures.  As the Council of Chalcedon expresses it, one nature works in communion with the other what is a property of each.  Therefore, Christ is our Mediator, Redeemer, King, High Priest, Head, Shepherd, and so on, not according to one nature only (whether it be the divine or the human), but according to both natures.
~BOC, FSD, VIII, 46-47