Catholics often argue (much like atheists, which should tell you everything you need to know about the quality of the argument I'm about to describe) that the Bible simply cannot be sufficient for Christian doctrine, because there are anywhere from 10,000 to 60,000 different Protestant denominations (and that number seems to get bigger every time). Therefore, we need an authoritative Church to tell us what the Bible really means. To simply study it is not enough.
The key to refuting this argument is to understand what a Catholic means when they say "Protestant". "Protestants" are not defined by what they believe, but what they do not believe. Put simply, a Protestant is a non-Catholic follower of any religion that would be considered "Christianity" by the secular world. If literally anyone says "I believe in Jesus", yet is not a member of the Catholic or Orthodox Churches, they are considered a "Protestant" (and Catholics often refer to the Orthodox Churches as "the original Protestants").
It is this ridiculously ambiguous definition that enables Catholics to exaggerate the number of "Protestant" denominations. Usually, the word "Protestant" is where they end it. They just say "there are (insert grossly exaggerated number here) Protestant denominations", yet it is extremely rare for even a single example to be cited. One meme I once received from a Catholic is an exception.
The meme names several denominations which, supposedly, believe that "the Bible is the only and single truth of God", and yet contradict each other.
Now, I am not entirely familiar with all of the denominations listed. In fact, I'm not even sure which denominations the first three abbreviations refer to, and frankly, I am enjoying my ignorance. Mormonism, however, I am very much familiar with, and I can tell you for a fact that Mormons do not believe the Bible is the only and single truth of God. As a matter of fact, the name "Mormonism" comes from their most important scripture: The Book of Mormon. On top of this, they also have Doctrine and Covenants, and the Pearl of Great Price.
Concerning the Book of Mormon, Joseph Smith said "I told the brethren that the Book of Mormon was the most correct of any book on earth, and the keystone of our religion, and a man would get nearer to God by abiding by its precepts, than by any other book." (History of the Church, 4:461). Joseph Smith, of course, was the founder of Mormonism, so his opinion rather counts for something...
Furthermore, the second prophet of Mormonism, Brigham Young, once wrote "I have never yet preached a sermon and sent it out to the children of men, that they may not call scripture." (Journal of Discourses 13:95). Modern Mormons deny this, sticking to the three scriptures previously mentioned, but the fact remains that he said it.
To make matters worse, the Book of Mormon itself says, in 2 Nephi 29:6, "Thou fool, that shall say: A Bible, we have got a Bible, and we need no more Bible." Mormonism, much like Catholicism, also has living human leaders. This includes the current prophet, Russell M. Nelson, who of course has the authority to declare Mormon doctrine. In the Pearl of Great Price, we read "We believe all that God has revealed, all that He does now reveal, and we believe that He will yet reveal many great and important things pertaining to the Kingdom of God." (Articles of Faith 1:9).
This is all basic Mormon doctrine. No one who knows anything about Mormonism could claim that Mormons believe the Bible is the only and single truth of God. The same could be said about Jehovah's Witnesses, who are also mentioned in the meme. Rather than believing the Bible is the single and only truth of God, Jehovah's Witnesses are required to believe "...the Bible is an organizational book and belongs to the Christian congregation as an organization, not to individuals, regardless of how sincerely they may believe that they can interpret the Bible." (The Watchtower, Oct. 1, 1967, p. 587).
Now, who does that sound like? Because to me, it sounds like rather than believing "the Bible is the only and single truth of God", JWs are actually taught to believe their Church has the sole authority to interpret the Bible. Just. Like. Catholics.
Now you see why Catholic apologists don't typically clarify what they include in the alleged 40,000 "Protestant" denominations. It's because in reality, the whole reason so many denominations exist is because they do not all hold to the Bible. The ones that do are usually distinguished by such trivial differences, including the typical day on which church services are held, that it's almost foolish to consider them separate denominations. When Catholics admit this (and occasionally they do), the argument disappears completely.
The argument we have examined today commits what is called the fallacy of equivocation. That is, the use of ambiguous language to either mislead the audience, or avoid commitment to an argument or position. Once the Catholic does commit by explaining exactly what they mean by "Protestant", we see that not all "Protestant" denominations agree with the Bible at all. A large number of "Protestant" denominations exist for the same reason Catholicism does. Rather than putting their faith in the word of God, they instead place there faith in the word of man. Fallible, sinful man. Luke 6:46-50 tells us why this is a terrible idea.