Adiaphora in Christianity refer to matters not regarded as essential to faith, but nevertheless as permissible for Christians or allowed in church. What is specifically considered adiaphora depends on the specific theology in view.
“I know,” says Paul, “that there is nothing unclean of itself,” (by unclean meaning unholy;) “but to him that esteemeth any thing to be unclean, to him it is unclean,” (Rom. 14: 14.) By these words he makes all external things subject to our liberty, provided the nature of that liberty approves itself to our minds as before God. ~John Calvin
It is, however, to be carefully observed, that Christian liberty is in all its parts a spiritual matter, the whole force of which consists in giving peace to trembling consciences, whether they are anxious and disquieted as to the forgiveness of sins, or as to whether their imperfect works, polluted by the infirmities of the flesh, are pleasing to God, or are perplexed as to the use of things indifferent. It is, therefore, perversely interpreted by those who use it as a cloak for their lusts, that they may licentiously abuse the good gifts of God, or who think there is no liberty unless it is used in the presence of men, and, accordingly, in using it pay no regard to their weak brethren. Under this head, the sins of the present age are more numerous. For there is scarcely any one whose means allow him to live sumptuously, who does not delight in feasting, and dress, and the luxurious grandeur of his house, who wishes not to surpass his neighbor in every kind of delicacy, and does not plume himself amazingly on his splendor. And all these things are defended under the pretext of Christian liberty. They say they are things indifferent: I admit it, provided they are used indifferently. But when they are too eagerly longed for, when they are proudly boasted of, when they are indulged in luxurious profusion, things which otherwise were in themselves lawful are certainly defiled by these vices.
Article here, by Gregory Koukl, This one talks about; Is there a reliable way to know when someone is restricting your Christian liberty?
And Martin Luther chimes in about it here:
That I may open then an easier way for the ignorant—for these alone I am trying to serve—I first lay down these two propositions, concerning spiritual liberty and servitude:—
A Christian man is the most free lord of all, and subject to none; a Christian man is the most dutiful servant of all, and subject to every one.
I liked this pdf on the subject: [It is easy to read and informative]
What Is It?
The freedom from God to do whatever you wish in any matter the Bible
does not address, without fear of persecution by the church or saints!
What Is It Not?
It is not an excuse to break any Bible principle, compromise your
conscience, put yourself in bondage, raise arguments in the church, harm
the reputation of the gospel, or destroy another saint’s walk with God!“Hast thou faith? have it to thyself before God. Happy is he that condemneth not himself in that thing which he alloweth.” Romans 14:22 “But take heed lest by any means this liberty of yours become a stumblingblock to them that are weak.” I Corinthians 8:9 ~~~
The Weak and the Strong
1 Accept the one whose faith is weak, without quarreling over disputable matters. 2 One person’s faith allows them to eat anything, but another, whose faith is weak, eats only vegetables. 3 The one who eats everything must not treat with contempt the one who does not, and the one who does not eat everything must not judge the one who does, for God has accepted them. 4 Who are you to judge someone else’s servant? To their own master, servants stand or fall. And they will stand, for the Lord is able to make them stand.
5 One person considers one day more sacred than another; another considers every day alike. Each of them should be fully convinced in their own mind. 6 Whoever regards one day as special does so to the Lord. Whoever eats meat does so to the Lord, for they give thanks to God; and whoever abstains does so to the Lord and gives thanks to God. 7 For none of us lives for ourselves alone, and none of us dies for ourselves alone. 8 If we live, we live for the Lord; and if we die, we die for the Lord. So, whether we live or die, we belong to the Lord. 9 For this very reason, Christ died and returned to life so that he might be the Lord of both the dead and the living.
10 You, then, why do you judge your brother or sister[a]? Or why do you treat them with contempt? For we will all stand before God’s judgment seat. 11 It is written:
“‘As surely as I live,’ says the Lord,
‘every knee will bow before me;
every tongue will acknowledge God.’”[b]
12 So then, each of us will give an account of ourselves to God.
13 Therefore let us stop passing judgment on one another. Instead, make up your mind not to put any stumbling block or obstacle in the way of a brother or sister. 14 I am convinced, being fully persuaded in the Lord Jesus, that nothing is unclean in itself. But if anyone regards something as unclean, then for that person it is unclean. 15 If your brother or sister is distressed because of what you eat, you are no longer acting in love. Do not by your eating destroy someone for whom Christ died. 16 Therefore do not let what you know is good be spoken of as evil. 17 For the kingdom of God is not a matter of eating and drinking, but of righteousness, peace and joy in the Holy Spirit, 18 because anyone who serves Christ in this way is pleasing to God and receives human approval.
19 Let us therefore make every effort to do what leads to peace and to mutual edification. 20 Do not destroy the work of God for the sake of food. All food is clean, but it is wrong for a person to eat anything that causes someone else to stumble. 21 It is better not to eat meat or drink wine or to do anything else that will cause your brother or sister to fall.
22 So whatever you believe about these things keep between yourself and God. Blessed is the one who does not condemn himself by what he approves. 23 But whoever has doubts is condemned if they eat, because their eating is not from faith; and everything that does not come from faith is sin.
This article starts this way:
In our discussion of Christian liberty there are a number of important topics that need to be considered such as: the liberty that Christ has purchased for believers; liberty of conscience; liberty and the weaker brother, avoiding the abuse of liberty; and, liberty and society or how true freedom in a nation is rooted in biblical Christianity.
and ends this way:
The only true source of liberty on planet earth is the liberty bestowed by Jesus Christ. It is a liberty rooted in His redemptive work. There is freedom from the guilt and penalty of sin as well as from sin’s power. There is the liberty under Christ’s law word from the doctrines and commandments of sinful men. And there is freedom from the Old Covenant ceremonial observances.
It is important for believers to understand that the liberty that we have in Jesus must never be used in a manner which hurts a brother in Christ. Liberty must be tempered by love for the peace and edification of the body. There are a number of peripheral doctrinal matters over which under strict conditions both the strong and the weak must not quarrel over or disturb the peace of the church. The strong must fully accept the weak as brothers and the weak must not judge the strong. Both parties must work together to advance Christ’s for God’ glory. Paul brilliantly rejects human autonomy in ethics while recognizing and dealing with the fact that sanctification is progressive; that it is inevitable that minor doctrinal disagreements will occur. May God give modern Reformed churches the wisdom and ability to fight for the fundamental doctrines of the faith while they love, nurture and protect the weak in the flock of Christ.