They believe that unity is about the core and center of what Scripture says -- a gospel reductionism which finds facts less compelling than the call to love. Indeed, as Norman Nagel pointed out, love is both the summary of the Gospel and the Law. It is hard to know which love modern Christianity is referencing in its summary -- whether Law or Gospel. But it is clear that everything else is negotiable -- only love is essential.
The problem is this. When you reject the parts you find objectionable or negotiable, you are probably also rejecting the very Word of God itself. It is official in most Christian traditions that no one has the power to change the truth of God's own self-disclosure or to reject it in part or in whole -- without also rejecting Christ. Indeed, somewhere in our official statements, all Christian traditions insist that both ministers and the faithful are no more and no less than servants and defenders of the apostolic Word and the prophetic promise. In reality, however, the practice has been to affirm words but to allow wide diversity in what those words are deemed to mean and to say. Some traditions are more able to hold the line than others but every Christian tradition suffers from a fear and reticence to condemn anything but the most egregious examples of heresy and apostasy. Holding to the core is enough while allowing diversity about the fringe.
The problem is this. Rejecting what some consider to be the fringe is actually attacking the core and degrading what Scripture says about that core because it raises questions about those fringes (even when Scripture speaks clearly and offends the modern view of things). Looking at the creed it is easy to see that rejecting God's deliberate creation of all that exists requires rejecting the truthfulness of the Word of God. Rejecting such things as the Virgin Birth or the physical resurrection of Jesus will inevitably lead to rejecting Jesus and all that has been said about Him and He has affirmed within the Scriptures.
We are not rejecting doctrinal formulations of men or of churches nor are we rejecting parts not essential to the whole. The modern confession has rejected the very Word of God itself, that which was entrusted to the apostles and the sacred deposit to be believed and treasured and witnessed. We do not know better than the Lord Himself. What we have concluded on the basis of observation and the conclusion of reason lies beneath God's Word and not above it. The divine revelation, as we know it in Scripture, is given to us to believe and therefore we submit to that Word and obey it. We stand under the Word of God and not above it.
What the Church has wisely considered open questions are those things where the mystery remains untouched by human wisdom and speculation so that nothing can be said dogmatically. They are on the fringes and are open not because we have decided they do not matter but because God has not disclosed them definitively. Unlike modernity which presumes that most of Scripture is unclear, we affirm that what is unclear is not simply little but the little which does not affect our faith and salvation. In this way we are not putting doubt on what God has said but refusing to put an explanation point where God has not placed one. Neither have we rejected what God has said in His Word in favor of another voice and another word that fits our moon in the moment. As soon as modern Christianity begins to omit from the faith the Scriptures that it finds uncomfortable or consigning those to the fringes where disagreement is not only tolerated but celebrated, the core and center of Scripture and the Christian faith is weakened and faith itself is at risk.