First level theological issues would include those doctrines most central and essential to the Christian faith… Denial of these doctrines represents nothing less than an eventual denial of Christianity itself… The set of second-order doctrines is distinguished from the first-order set by the fact that believing Christians may disagree on the second-order issues, though this disagreement will create significant boundaries between believers… Third-order issues are doctrines over which Christians may disagree and remain in close fellowship, even within local congregations. I would put most debates over eschatology, for example, in this category… Christians should never separate from a church over third-order issues.
Albert Mohler