The alleged "Great Apostasy" is a foundational claim of the Mormon religion. According to Mormonism, shortly after the death of the apostles*, the entire Church apostatised, leaving every church corrupt and without the Gospel, until Joseph Smith arrived. If the Great Apostasy never happened, there was no need for Joseph Smith to restore the Gospel, and the entire Mormon religion would be a massive fake.
Of course, in Christianity, such a complete apostasy is impossible. Jesus promised that the gates of Hades would never overcome the Church (Matthew 16:18). Even if a zombie apocalypse could wipe out 99.9% of the human race, it is guaranteed that a significant portion of the remaining .1% would be true Christians. But the Mormon god is not so powerful. He proved incapable of preserving his word for more than a century, losing an entire religion and leaving the planet without his gospel for 1,700 years! How can such a useless god be expected to preserve the restored gospel from 1830 - 2021?
As it turns out, the Mormonism of the modern day is significantly different from the Mormonism of 1830. Even the Book of Mormon, which Joseph Smith once called "the most correct book of any on earth" and the keystone of the Mormon religion (1), has undergone more than 4,000 changes since its original publication. Unlike "changes" in the Bible, which can hardly even be described as changes so much as minor translation issues with no impact on doctrine, the changes in the Book of Mormon are significant. The words "Son of" have been frequently inserted to sentences which initially spoke of "the everlasting God" (e.g. 1 Nephi 11:32). Identities of people have been changed (e.g. Mosiah 21:28). Entire phrases have been removed (e.g. 3 Nephi 16:10). If changes of such magnitude had occurred in even one version of the Bible, Mormons would use it as evidence of apostasy, and yet such changes are tolerated in the Book of Mormon for reasons I simply cannot fathom.
Aside from changes to the Mormon religion that occurred after the death of Joseph Smith, Smith himself made a few changes to his religion that are quite significant. For example, the Book of Mormon, even with its many changes, has always been a monotheistic work. Despite supposedly containing the "fullness of the Gospel", the henotheistic (the belief in the existence of multiple gods with special attention being paid to one) nature of the Mormon gospel is nowhere to be found within. In fact, the Book of Mormon very closely resembles the Christian doctrine of Trinitarian monotheism. Alma 11:22-31, for example, contains an account of an angelically inspired man named Amulek, who testifies that there are no other gods. Alma 11:44 declares that the Father, Son and Holy Spirit are one eternal God. Mormon 7:7 again affirms that the Father, Son and Holy Spirit are one God. Clearly, therefore, Smith's original understanding of the Gospel was quite different from the henotheism he later went on to preach, and more closely resembled the true Christianity he was attempting to corrupt.
All of this ultimately means that the Mormon claim of a great apostasy following the death of the apostles leaves Mormonism open to the possibility of another great apostasy lead by none other than Joseph Smith himself! Whereas the Christian God is too powerful to be phased by even the best demonic attempt to threaten His Gospel or remove His Church from the Earth, the Mormon religion has historically been corrupted and mutilated by its own prophets! This isn't even some unverifiable claim that was made up to form the basis of Christianity against Mormonism. It is a historically verifiable fact that modern Mormonism looks very different from the original Mormonism, up to and including major differences in the Book of Mormon. This leads to only two possible conclusions:
1) God is unbelievably inept, being unable to preserve his message for more than a few years, and is absolutely abysmal at choosing good prophets.
2) The Gospel was never lost in the first place, needed no restoration and Joseph Smith is a false prophet.
Both of these possibilities lead us to conclude that the Mormon God is not worth worshipping, and the Mormon religion is more corrupt than the Democrat party. This would be a good time for Mormons to abandon Joseph Smith in droves, instead worshipping the one true God of the Bible.
*As an interesting side note, 3 Nephi 28:7-8 tells us that John the apostle will never die, meaning he should still be alive today, and should have both been able to help preserve the original gospel, and should be able to testify to the Mormon gospel to this very day.
References
1. Smith, Joseph - History of the Church, 4:461.