Comment by Joe Heschmeyer over at his blog:
"...I eventually realized something much more fundamental: either the Holy Spirit guides the Church or He doesn't.
If He does, then I don't have to worry about false dogmatic definitions. I can just trust the Church and treat her as my Mother. If He doesn't, then the Marian doctrines are the least of my worries. So if we can trust the Church on the Trinity and Christ's Dual Natures, we can trust Her on the Immaculate Conception and Assumption. And if we can't trust her on the Immaculate Conception, we can't trust Her on the more basic Christian doctrines.
So I ultimately came to realize that if the Holy Spirit does guide the Church, Catholicism is true. But if He doesn't, it would be much more than Catholicism proven false -- all of Christianity would be untrustworthy. Fortunately, the evidence is rather overwhelming (at least in my opinion) that the Holy Spirit does actively guide and lead the Church."
Well said, Joe.