I'll tell you why we do this.
This is like being in a fencing duel a la "The Three Musketeers" with several men to a side.
Touche! Riposte! (re-post? )
Seriously, yeah, Stanley said what I was trying to say, and as usual, did it better. It's good for my humility! (Tho' bad for his )
These things DO matter, and as soon as you try to talk about unity, let's talk about unity under a device that does not insist on doctrines that to some of us are key (salvific), such as OSAS (Once Saved, Always Saved - that when we say the sinner's prayer one time, BOOM! SHAZAM! we are saved forever and it's irrevocable) or Sola Scriptura, that expects us to deny the rest of Holy Tradition (as something distinct from local church practices) as being authoritative.
Not meaning to place anyone here as malicious, but I am reminded of Theoden's exchange with Saruman at Orthanc: "Shall we have peace?"
Respectabiggle, again, your question of why we keep bringing up the question of authority is good - the answer is simply that it is the first question, and all disagreements spring from it. The only hope of dealing with the differences, which if we examine what they are, we agree are critical, is to get as close to the root of the problem as possible. This is precisely what Lewis didn't do, and it is where his usefulness to us ends.
This kind of question may not be helpful to an atheist who isn't ready to consider Christianity, but it sticks out like a sore thumb for anyone who has progressed to the point that it does become relevant.
"Eh? Two views? There are a dozen views about everything until you know the answer. Then there's never more than one."
Bill "The Blizzard" Hingest - That Hideous Strength