What evidence have you of this?
History testifies otherwise. The early Church, even contemporaries of the Apostles, appointed and trained by them and practicing during their lives (like Clement and Ignatius of Antioch), hold to Catholic doctrines and interpretations, oppose Gnostic (and other heresies), and show virtually no evidence of the novel doctrines that Joseph Smith preaches.
and as God has done in the past, he allowed them to be lost and forgotten.
The Church preserved records of the many heresies it did confront, so it’s not like it was busy trying to eliminate all references to things it opposed. And we have the testimony of the Scriptures that I referenced in Post 2 about how God’s Word cannot be expunged from this earth, so even the idea that some group could totally suppress such elements of God’s Word is false.
Thus, we should see some record of Smith’s novel teachings amidst the early church. We don’t. They are new to Smith, invented by him (or plagiarized or borrowed and modified from Smith’s own contemporaries, which he did quite often).
Moreover, when has God “allowed [His truths] to be lost and forgotten?” He has always maintained a faithful, authoritative covenant line, always kept His promises and preserved both His covenant and His laws. Even when people weren’t following them.
Here’s a sample record:
Adam and Eve: sinned, exiled from the Garden, but still with knowledge of and relationship to God, as seen in their family (passing this on to children).
Noah: though he and his family remained the only faithful among mankind, God preserved them and continued the line of His faithful through Noah (notably down through Shem–whom the Jews and many scholars even today believed to have transmitted authority through to Melchizedek, if Shem was not actually Melchizedek himself).
Abraham: God calls forth the Father of Faith into a new covenant, choosing His own people and creating a continuing covenant that is now even easier to follow, for the records kept.
The history of Israel: through many apostasies, heresies, abominable practices, sin, etc, even the division of the kingdom and separation of the 10 tribes, we still find that God has maintained a continuous Covenant People, with authority, right down to Jesus’s day. In fact, God has even continued to keep His promises to the Jews and preserved them as a people and in their faith even after most of them rejected His own Son.
The evidence of God’s working in Salvation History to always preserve a visible, recognizable, authoritative line of covenant people shows:
The total impossibility of the complete corruption or loss of His Word (thus countering the first premise in the OP and all that follows from it);
The complete lie of any idea of a total apostasy–indeed, even the massive blasphemy of such a lie, that never before has God left His people orphaned, but now, AFTER the Incarnation, Death, Resurrection, and Redemptive work of His own Son AND the, for the first time, granting of the Holy Spirit to the Church God now totally abandons His people for 1260+ years, in defiance to all His promises and words, and for the first time in history (i.e., despite never doing so under prior, inferior, pre-redemptive, pre-Holy Spirit covenants).
It is the height of blasphemy to accuse God of such crimes, and seeks to unmake Him by defying His trustworthiness, truthfulness, benevolence, and Providence. There are few more insidious and diabolical strategies for undermining faith and relationship with the One True God.
This is why Satan tries so hard to instill such blasphemies in the mind of man, dividing God’s children and confusing them, making them believe such terrible things about God even while they think they are honoring Him.