“In essentials, unity; in non-essentials, liberty; in all things, charity.”
Certainly, all Christians can agree on the clear scriptural teaching that charity should be shown in all cases, to all people, in all circumstances. We are not only to speak the truth, but to speak it in love, gentleness, and humility (1 Pet 3:15-16).
However, the formula above, tracable to Augustine, has over-promised and under-delivered for centuries. Of course, the medieval church was a lot less complicated: just believe whatever the church teaches. (No mortal can possibly know everything the church teaches, so you just sign a blank check: it’s called “implicit faith.”) Not on the basis of the church’s authority, but on the basis of Scripture’s inherent clarity, the churches of the magisterial Reformation regard the ecumenical creeds and distinctive confessions and catechisms as faithful summaries of the “essentials.” So for Rome the “essentials” are whatever the church requires for salvation, while Lutheran and Reformed churches confess collectively what they believe the Bible to teach as essential for sound faith and practice.
However, the famous maxim above has been invoked more in radical Protestantism than in Roman Catholic or Lutheran and Reformed traditions. By “radical Protestantism” I mean the trajectory that runs from Anabaptist to Quaker to Restorationist sects in the New World. Although an English Puritan (that is, a minister in the Church of England who would not conform to episcopacy), Richard Baxter (1615-91) alarmed many fellow Puritans by revising and rejecting key features of Reformed theology. Baxter first labeled as “mere Christianity” turned out to be another set of doctrines that were diametirically opposed to the confessions he had subscribed. Many today think that Baxter is the original source of our maxim, because he appealed to it to justify his program. Where churches had agreed in representative assemblies on what Scripture teaches, Baxter’s spirit is in many ways that of modern evangelicals. “Essentials” are whatever I think they are; church dogmas are the letter without the Spirit. The most important doctrines are the ones that all professing Christians can agree to, which of course leaves out doctrines like original sin and the sovereignty of God’s grace in election, justification, the new birth, and preservation of the saints. Professing Christians disagree over the sacrifice of the mass, purgatory, merits, and ultimate authority in the church, so these matters must also be excluded from any list of essentials—or “mere Christianity.”