Triage and Cooperation in the Local Church (Part I)
In 1627, German Lutheran Theologian, Rupertus Meldenius, penned a tract on Christian Unity. In it, he writes, "In essentials, unity; in nonessentials, liberty; in all things, charity." These words provide a framework for how believers can respond to one another when we disagree. Meldinius's words are a distant echo from the Scriptures, which call believers to maintain unity and not quarrel over opinions. The temptation is to believe that all doctrines are of equal weight and significance. Doctrinal distinctions can be applied as one would engage in a practice of triage.
The origin of the word triage is French and means "to sort."  The word's definition is as follows: the sorting of and allocation of treatment to patients and especially battle and disaster victims according to a system of priorities designed to maximize the number of survivors. The sorting of patients (as in an emergency room) according to the urgency of their need for care  When applied to theology in the local church in relation to cooperation in the kingdom of God, it addresses what we should prioritize in gospel ministry. In his article, "A Call for Theological Triage and Christian Maturity, Al Mohler links the idea of medical triage to theological triage. He writes, "The same discipline that brings order to the hectic arena of the Emergency Room can also offer great assistance to Christians defending truth in the present age…A discipline of theological triage would require Christians to determine a scale of theological urgency that would correspond to the medical world's framework for medical priority. 
The importance of developing a framework to filter theological issues is essential. At least four categories are defined in two articles by Al Mohler and Gavin Ortlund that help us understand theological triage. In other words, when an issue is presented, we must decide which category fits is best.
The first category is regarded as a primary category for first rank theological issues. These are doctrines considered to be essential to the Gospel. They are worth fighting for because, if they are mishandled, the integrity of the Gospel could be compromised. Many consider doctrine in this category to be a fault line belief and error. They mark a definite line between orthodox and non-orthodox beliefs. Some doctrines that are the first rank are as follows: Substitutionary atonement, Trinity, the deity of Christ, the humanity of Christ, the authority of Scripture, and justification by faith. These are examples of first-rank doctrines that must be defended and maintained to establish clarity of the Gospel. Cooperation with other believers and fellowship with other local churches who hold to the first-rank doctrines is essential to Kingdom work progressing.
The second category is reserved for secondary theological matters. Gavin Ortlund describes these as "Doctrines that are urgent for the health and practice of the church, such that Christians commonly divide denominationally over them…" Believers may be divided into local assemblies or denominations by these secondary differences. Some examples of secondary doctrine could be Church Governance, the Role of Women in Ministry, Modes of Baptism, or communion approaches. Application of secondary matters does lead to each local church worshipping differently. Cooperation between believers is vital at this level. Churches can still do many things together, even though they will engage in secondary church matters differently. For example, churches can participate in area-wide revivals with those who hold to the same primary doctrines, but they may have a hard time with a joint class on baptism or church membership.
The third category is tertiary. These are matters two times removed from what is essential to the integrity of the Gospel. They are lesser in significance than the secondary issues of theological agreement. For example, in category 1, the authority of the Scriptures is an essential or first-rank issue. We must defend the inerrancy of Scripture at all costs. However, in this third category, the way we view the Book of Revelation's specific outcomes is tertiary. In other words, Believers can disagree on views of the tribulation and millennium and still go to the same church and remain in close fellowship with one another.
The last category is called the adiaphora category. This is a word that means "things indifferent."These theological matters are neither commanded nor forbidden in the Scriptures. An example of a fourth-rank issue is the musical instrumentation in a worship service. These may be relevant and intellectually stimulating but not theologically significant.
These categories can act as filters to the framework of theological triage. Not every hill is a hill to die on, but some are worth the struggle.
Some Resources on Theological Triage and the Local Church
i. When Should Doctrine Divide?” By Gavin Ortlund
ii. “A Call for Theological Triage and Christian Maturity” By Al Mohler
iii. In Essentials Unity, In Non-Essentials Liberty, In All Things Charity” By Mark Ross
i. Finding the Right Hills to Die on by Gavin Ortlund
ii. Uncommon Ground by Tim Keller and John Inazu
 https://www.ligonier.org/learn/articles/essentials-unity-non-essentials-liberty-all-things/  https://www.dictionary.com/browse/triage  https://www.merriam-webster.com/dictionary/triage  https://albertmohler.com/2004/05/20/a-call-for-theological-triage-and-christian-maturity-2  Gavin Ortlund, Finding the Right Hills to Die On (The Gospel Coalition) (pp. 12-14). Crossway. Kindle Edition.  Gavin Ortlund. Finding the Right Hills to Die On (The Gospel Coalition) (pp. 19-20). Crossway. Kindle Edition.