Some people appear to be astonished that Andrew Sullivan thinks that Mormons aren't Christian (or, at best, are a decidedly bizarre brand of Christian). I'm astonished that anyone is astonished; it is always dangerous to assume that someone who dissents from orthodoxy on issue X will necessarily do so on issue Y. Of course, being ruled out of bounds is an existential hazard. Speaking as a Jew whose upbringing was somewhere between Reform and Reconstructionist, I know that some more Orthodox sorts regard my claims to Jewishness as, well, problematic. (It is possible to survive such denunciations with reasonable equanimity.) And, of course, there are the secular versions of this particular game. Are you really a conservative? A leftist? A vegetarian? An English professor?
As it happens, this spat cropped up just as I started organizing my Victorian anti-Catholic sermons for this project. It will come as no surprise that the preachers expend quite a bit of effort on demonstrating that Roman Catholicism is--you guessed it--not Christian. Contemporary fundamentalists, some of whom depend heavily on Victorian writers for their theological arsenal, would not disagree with that assessment of the situation; in fact, some of them regard Mormonism and Catholicism as analogical heresies. In the Victorian period, this kind of attack-by-analogy (or, for that matter, self-defense by analogy) could twist writers into unintentional pretzels. For example, Protestants commonly argued that Roman Catholicism and Judaism shared a number of theological errors; in fact, evangelical Anglicans sometimes called Anglo-Catholics "Judaizers" (an epithet with a much longer history). This argument frequently led Protestants to simply read Judaism in the light of Roman Catholicism, or vice-versa, with comical results. (A rabbi is not a Catholic priest. Really.) At the same time, Protestants also argued that Jews were naturally antagonistic to Catholicism, given Jewish prohibitions against idolatry. When the same writer tried to squeeze both positions into one text, the result was not entirely coherent.
Like their sixteenth- and seventeenth-century predecessors, Victorian Protestants argued that Roman Catholicism committed sins of addition. Most of my readers are probably familiar with the claim that the Catholic Church simply added pagan survivals to "pure" beliefs; the Rev. John Gore Tipper, for example, argues that the Papal system originated with Semiramis somewhere around 1900 BC--another theory that's still with us, as the link notes--and goes on to "prove" that the cross on the priest's vestments refers to Tammuz ! But preachers usually claimed that all of Roman Catholicism fell under the heading of a false addition to the original truths. Some more examples from my notes:
Under the baneful shadow of that Church’s influence and the usurped dominion of her head, the Bishop of Rome, this our Church of England had for ages suffered and groaned, heavy laden with the spiritual fetters which she had forged and diseased with the corruptions which she had fostered; at length through GOD’s mercy the light of the full truth of His Gospel shone out upon us now three hundred years ago: in the light of His open volume of instruction, and of the recorded teaching of primitive ages of the Church’s purity, we saw our errors, we learned to separate the precious from the vile, we retained our Catholic standing, and the great Catholic verities, the primitive truths and practices of the Church of CHRIST, as sanctioned by GOD’s Word, we removed those incrustations of deadly error under which they were well-nighed buried; we threw off that yoke of allegiance to the Bishop of Rome, to which by mere usurpation we had become enthralled, and the continuance of which would have been the means of perpetrating error—and we remained as we had been from the beginning THE CATHOLIC CHURCH OF ENGLAND, now reformed from uncatholic and pernicious errors, against which we have ever since continued in doctrine and practice to protest. 
But now we must be allowed briefly to explain the principles on which this deception of souls takes effect—the means by which it is made available. They resolve themselves, we conceive, into two—an imposition and an injustice—an act of tyranny and a claim of falsehood. The imposition consists in arrogating the name and authority of THE OLD—THE ONLY TRUE church. The advocates of Popery run over the promise made to the Church Catholic: they assume or take for granted that the Church Catholic is the Church of Rome, and so claim the promises for themselves. Though every Epistle almost repudiates the notion—though history systematically detests the mis-statement; yet reiterated assertion passes for cogent argument, and assumption is substituted for truth. Then antiquity is advanced as a plea for attachment. Now, it is the novelties of Popery against which we are bound to protest. The primitive Nicene Creed we accept—it is the modern additions of Pope Pius IV. that we condemn and reject. 
These are the principles, at once of primitive Christianity and of the English Reformation. We invented nothing. We added nothing. Nay, we altered nothing. We did but clear away the accumulated rubbish of the middle ages, and reach and rest upon the old foundations. 
Roman Catholicism, although old, becomes "new," while Protestantism, although new, becomes "old." I've discussed this before.
 Rev. John Gore Tipper, M.A., A Sermon Preached in St. Stephen’s Church, Canterbury, on Sunday Morning, February 10, 1867, Being the Day on
which the attention of the Congregations throughout the Parish of Islington was
directed to the subject of Ultra-Ritualism (London: Hatchard and Co., 1867), 7, 11-12.
 Rev. Nugent Wade, M.A., Romish Aggression: The Spirit in Which We Should Resist It. A Sermon Preached in the Parish Church of St. Anne, Westminster, on Sunday, November 17...To Which are Appended the Parochial Addresses to the Queen and the Bishop of London: Together with His Lordship's Reply (London: John Masters, 1850), 7.
 Rev. J. Richardson, B.A., Popery a Deceiver of Souls. A Sermon, Delivered in St. James' Church, Preston, on Thursday Evening, April 24, 1851 (Preston: H. C. Barton, 1851), 18.
 Rev. Hugh M'Neile, D.D., The English Reformation, a Re-Assertion of Primitive Christianity. A Sermon, Preached in Christ Church, Newgate Street, on the 17th of November, 1858, the Tercentenary Commemoration of the Accession of Queen Elizabeth, 2nd ed. (Liverpool: Adam Holden; London: Longman, Green, Longman, & Roberts, 1858), 39.