If President Monson were guilty of apostasy—which I think is essentially what you’re saying—he would face a disciplinary council, just like anyone else. As for what the council would determine, that would be wholly dependent upon what he had said or done—but I’m guessing with as much knowledge as he should have, they’d be pretty strict. HTH! ETA: I’ve answered again, based on the ensuing discussion. Enjoy: http://www.answerbag.com/a_view/6390661

If I didn't believe Monson to be a true Prophet, I would raise my hand to oppose during the sustaining vote of the next ward / stake / general conference.

If there are legitimate concerns his peers, which is to say the Apostles (The Church president is traditionally the senior apostle, calculated by time served in the Twelve, not by age), have the authority and responsibility to make such judgments, and to place another in his place if deemed necessary. As Otter explained earlier, one can refuse to sustain and would be encouraged to discuss concerns with local leadership, but ultimately such matters are the concern of the presiding quorums of the Church. An individual member who presumes to actively campaign against church leadership would himself be subject to church discipline. Lots of people clam Monson is a false prophet, just as lots of people claim Pope Benedict is evil, just as lots of people claim <insert religious leader of choice> is a hypocrite. Big deal.

Okay… while I stand by my original answer (http://www.answerbag.com/a_view/6363789), the discussion following laie_techie’s answer (http://www.answerbag.com/a_view/6366215) has led me to understand that Martureo is looking for a much deeper response than I originally gave. As such, here’s another crack at it: There are really two theoretical possibilities, here: either a) President Monson has always been a false prophet, and I’m just now figuring it out; and b) President Monson has recently stopped being a true prophet and become a false one. These two scenarios have *extremely* different ramifications, and thus must be dealt with separately. If we assume the latter scenario—that is, that President Monson has “fallen” from his sacred calling and is no longer worthy/able to continue in that position—I think that we’ve discussed the appropriate actions and procedures quite fully. If, however, we were to assume the former—that is, that President Monson were never a true prophet, to begin with—then I’ve got *much* bigger fish to fry than what old Tommy might or might not be. Allow me to explain: My testimony of the Gospel of Jesus Christ is based on revelation: a sure knowledge that He lives, and that the Church of Jesus Christ is indeed exactly what it claims to be (i.e. the only true and living Church upon the face of the Earth, continually presided over and guided by the Lord Himself). In short, I know these things through the “more sure word of prophecy” spoken of in 2 Peter 1:18-21; or to put it another way, I—like virtually every member of The Church of Jesus Christ of Latter-day Saints, and many others that aren’t members thereof—am what the Bible calls a “prophet” (see Revelation 19:10). So therein lies the issue: if I were to receive revelation declaring that President Monson has always been a false prophet, that would necessarily negate the revelations that I have already received, declaring him to be a true prophet. If this be the case, it therefore calls into question my own ability to receive revelation. If I can’t rely on my own ability to receive revelation, then my testimony of the Savior is based on a faulty premise—or at the very least, on shaky ground. Such were the thoughts that went through my head, a few months ago, when an unrelated incident placed me in a position of what psychologists call “cognitive dissonance,” and I remained there—unsure that God even existed—for a couple of weeks. The thing about cognitive dissonance is that competing evidences tend to come from multiple sources, which means that the only ways to eliminate the dissonance—creating “cognitive consonance”—are to either eliminate one of the pieces of information, or to amalgamate the information into a new understanding that includes all of them. This is what finally happened in the aforementioned situation: once I understood *why* the event that precipitated the dissonance occurred and was able to amalgamate it into the understanding that I already had, the dissonance was replaced by consonance and all was well again. This is likely why some people that leave a good organization due to arguments against it sometimes say they felt peaceful, after leaving: they have achieved cognitive consonance—that same consonance that would have been achieved, had they decided to stay. Contrast this to the scenario that you’re suggesting: both the revelation I’ve previously received—that President Monson is a true prophet of God—and the revelation I’ve now hypothetically received—that President Monson was never a true prophet of God—are in direct conflict. There’s no way I can think of to amalgamate those two pieces of information, mostly because they come from the same source. Thus, if one be invalidated, the other goes with it. So in short, I don’t what I’d do, beyond being in an horrific state of cognitive dissonance. Unless some additional information could come to light that allowed me to understand how President Monson could possibly be a true and a false prophet concurrently, I’d probably be forced to assume that personal revelation be a lie. If personal revelation be a lie, then all revelation—including scripture—goes with it. If all revelation be a lie, then God either doesn’t exist or doesn’t care about us. And if either of those cases be true, I’d probably have to become an atheist, which as Mister_IT has pointed out, is exactly what most former Latter-day Saints do. So… enough reading for you? ;-)

God would not permit a false prophet at the head of his church. Therefor a church with a false prophet at it's head would thereby be proven not to be God's. . I would continue to use the church as my general guide, as I have never found any other whose doctrine makes better sense or leads to greater happiness, but I would loose the assurance that it could be relied on.

Doctrinaly: President Monson was given authority from God by the laying on of hands to be the Prophet and lead the Church according to the will of God. If he were to decide to lead the church astray and ignore the counsils of God, God would remove him from his place. "The Lord will never permit me or any other man who stands as President of this Church to lead you astray. It is not in the programme. It is not in the mind of God. If I were to attempt that, the Lord would remove me out of my place, and so He will any other man who attempts to lead the children of men astray from the oracles of God and from their duty." President Wilford Woodruf (Sixty-first Semiannual General Conference of the Church, Monday, October 6, 1890, Salt Lake City, Utah. Reported in Deseret Evening News, October 11, 1890, p. 2.) So assuming the Church were true, as I believe, and President Monson became a fallen prophet this is what would happen. Now obviously if the church were false, hypothetically, then surely all the apostles and probably most of the other general authorities would have to be in on it and try to cover it up. Because thanks to the quote by President Woodruf a prophet leading the church astray would disprove the church. Personally: If I were sure President Monson were a false prophet I would become a temporary agnostic and begin my search for a true prophet of God, or whatever other means God was using to personally lead men on earth.