One troubling Mormon doctrine is the idea that our church is the only true church while all the others are an "abomination" in God's eyes, or at least they do not have a "fullness of truth." It's a fundamental aspect of the LDS faith that Joseph Smith was called by God to restore Jesus's original Church and authority to the earth after all the others had apostatized. Today, only in the LDS Church can there be found a fullness of God's truth, along with God's priesthood authority, necessary for the salvation of all people. This is why Mormons, to the annoyance of everyone, still try to convert other Christians. Sorry, but your religion isn't true enough.
The loophole in this doctrine, which Mormons are quick to point out, is that if you don't have a chance to hear about the Church in this life (which includes 99.99% of the 100 billion humans who have ever lived on earth) then you will get the chance in the next life to accept it. Mormons robust attempts to convert the living world pale in comparison to the missionary efforts supposedly going on among the dead. But even then, dead people have to accept the same truths that the LDS Church teaches among the living: in effect, they have to join the Mormon Church in the afterlife. This, of course, is why Mormons do temple work for dead people in temples.
There are many arguments against such a silly idea the Mormons are sole possessors of God's truth and power, but one of the most damaging is to just do the math and observe the incredible improbability that such a claim is true. Monotheism has only been around for 3 thousand years, but humans have lived on the earth for roughly 100 thousand years. Prior to monotheistic religions (the first being Judaism) humans lived in small clans, tribes, and chiefdoms which began to practice a pagan, animist, or nature-worship religious prototype about 30,000 years ago. These religions slowly evolved into polytheism which then evolved into monotheism during the Axial Age - when most other forms of religious and philosophical thought emerged in larger agricultural societies. Still, none of these religions are salvific yet. Christianity hasn't even been invented yet. It did not arrive on the scene until 2,000 years ago, and yet, there is no evidence that Jesus even wrote down his teachings, let alone established a Church, as Mormons claim.
Mormonism didn't come around until 1830 and today only boasts a membership of 13.5 million members. Despite it's claims to be the fastest growing Church in the world (not true), Mormons have a serious retention problem since only 30% of it's 13.5 million members are even active. Most Mormons are first generation converts and nearly 50% of these stop attending after the first year. Yet all these lapsed Mormons are still included in the 13.5 million member total. You are still counted as a statistical member even if you ask for your name to be removed from the records of the Church. Mormons only compose 1.6% of the U.S. population and their growth rate is decreasing, not increasing. None of this bodes well for a religion that is suppose to convert the world.
I list these boring statistics to emphasize the following point: out of the 100 billion humans who need to accept the gospel, as preached by Mormons, only a smattering of people have ever done so, the majority of these people do not even "endure to the end," and the Mormon growth rate is going in the wrong direction. Considering that Mormonism is only 1 of 10,000 distinct world religions (or 1 of nearly 30,000 distinct Christian subgroups of religion) the probability of Mormonism being the "one and only true Church" is extremely improbable. Most other religions, populated in higher numbers by sincere believers, also claim to be the only true church. So what are the chances that Mormonism is really God's one and true Kingdom on Earth? Not very high.
Finally, when you consider Mormon doctrine as it relates to cosmology, the prospect looks even more grim. For example, consider that Mormons believe that Jesus sacrificed himself vicariously not just for every person to ever live on this earth, but for every other "human" who has lived on all God's other planetary creations! Assuming that God created "the heavens and earth" (the observable universe) that would be 100 billion stars in our own galaxy and then 100 billion other galaxies - each with it's own 100 billion stars. Astronomers are rapidly discovering earth like planets orbiting many of the stars in the universe, so the number of life-sustaining planets could be extremely high. Could you imagine alien prophets telling their alien followers (by the way, there is no reason to assume aliens would even look human since humans are a product of a unique evolutionary process that occurred on our planet, but wouldn't necessarily occur on theirs) that there exists another alien, named Jesus, who died for their sins too? Our own earthling Jesus myth is hard enough to believe. Could you imagine how many aliens would have faith to believe? Even to Mormons, who believe some pretty strange things, this claim should stretch the limits of credulity.