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Question: What do we know about Orson F. Whitney's "Strength of the Mormon Position" claim regarding a Catholic theologian?
Orson F. Whitney quoting a Catholic doctor of divinity: "You Mormons are all ignoramuses. You don’t even know the strength of your own position"
Note: This article derives from a blog post made by FAIR member Kevin Barney. 
Many members know this quote from LeGrande Richard's book, A Marvelous Work and a Wonder.
Elder Richards quoted from a pamphlet written by Orson F. Whitney:
A Catholic Opinion.–Many years ago there came to Salt Lake City a learned doctor of divinity, a member of the Roman Catholic Church. I became well acquainted with him, and we conversed freely and frankly. A great scholar, with perhaps a dozen, languages at his tongue’s end, he seemed to know all about theology, law, literature, science and philosophy, and was never weary of displaying his vast erudition. One day he said to me: “You Mormons are all ignoramuses. You don’t even know the strength of your own position. It is so strong that there is only one other tenable in the whole Christian world, and that is the position of the Catholic Church. The issue is between Catholicism and Mormonism. If we are right, you are wrong; if you are right, we are wrong; and that’s all there is to it. The Protestants haven’t a leg to stand on. If we are wrong, they are wrong with us, for they were a part of us and went out from us; while if we are right, they are apostates whom we cut off long ago. If we really have, as we claim, the apostolic succession from St. Peter, there was no need for Joseph Smith and Mormonism; but if we have not that succession, then such a man as Joseph Smith was necessary, and Mormonism’s attitude is the only consistent one. It is either the perpetuation of the Gospel from ancient times, or the restoration of the Gospel in latter days.” 
No name is given in this source, but it has been located in Elder Whitney's autobiography:
The source is Orson F. Whitney’s autobiography, Through Memory’s Halls: The Life Story of Orson F. Whitney, as Told by Himself (Independence, MO: Zion’s Printing and Publishing Company, 1930), 222-23.
The Catholic theologian’s name is John M. Reiner.
This stance predates the Vatican II counter-reformation and may not reflect general Catholic sentiment.
- Kevin Barney, "A Footnote to 'The Strength of the Mormon Position'," bycommonconsent blog (16 January 2008).
- Orson F. Whitney, Saturday Night Thoughts, Part 3, (Salt Lake City: Deseret News Press, 1921), 63-64.