How do we know which things are disputable?

The Greek term, ἀδιάφορα (adiaphora, meaning "not differentiable") refers to those issues of faith and practice, as well as ethics and morality, which are not essential to the Christian faith, and are thus a matter of conscience for individual Christians (and by extension local churches and denominations).  In other words, when we're not talking about the essentials of our faith, (a typical definition of which might be the Nicene Creed and the authority of Scripture, plus salvation by grace through faith) we as Christians are free to agree to disagree without straining the bonds of Christian fellowship.  This is of course in theory, in practice things can get real messy and even violent {see: The Thirty Years War for a brutal example}.

Which leads to a fundamental question that should concern all Christians: How do we define what is disputable/debatable and what is not?  Striving for agreement on what is "essential" to our faith is helpful, but not nearly enough as we might disagree strongly about what ought to be on that list, an outside arbiter is necessary to help Christians keep their disputes in perspective.  The primary answer is rather simple in the abstract although often difficult in practice: The adiaphora are those things which are "neither commanded nor forbidden in the Word of God" (as the 1577 Formula of Concord puts it).  If the Word of God commands that we do something, it cannot be a matter of conscience for a Christian to choose to obey, we must do so.  If the Word of God forbids an action/attitude, it also cannot be a matter of conscience for a Christian to fail to obey, we must do so.  For example, does the Bible teach about marriage, divorce, extra-marital sex, or homosexual behavior?  It does indeed, in many places.  Therefore it is not for the Church, nor for individual Christians to choose whether or not they wish to obey in these areas, it is a matter of faithfulness to God, a requirement of discipleship.  Does the Bible teach about voting, Bible translations, music choices in worship, art/statues in our worship spaces, the viewing of movies/TV, or social media?  It does not, not directly.  Therefore it is incumbent upon the Church, and individual Christians, to apply Biblical principles (i.e among others: respect for Truth, the pursuit of purity, the Fruit of the Spirit) in these areas, following the example of Jesus and seeking the will of God as best we can in accordance with our God-given wisdom, our conscience, and the leading of the Holy Spirit.  

In the end, our list of essential ought to be shorter than our list of that which is disputable/debatable.  The core of Christianity we ought to be able to list on one piece of paper, and is not open to debate (although many have tried, historically and today as well).  At the same time, we ought to view each other with love and charity regarding those things about which we disagree which are not essential to our faith.

Treat fellow Christians with whom you disagree with love and charity?  Won't that shock the world.  What an amazing opportunity to show the Lost the transforming power of the Holy Spirit at work among the people of God.