Look at how much the Bible trusts itself.
6 The words of the Lord are pure words,
like silver refined in a furnace on the ground,
purified seven times.
7 You, O Lord, will keep them;
you will guard us from this generation forever.
8 The grass withers, the flower fades,
but the word of our God will stand forever.
New Testament authors continue this theme with verse like these.
18 For truly, I say to you, until heaven and earth pass away, not an iota, not a dot, will pass from the Law until all is accomplished.
31 Heaven and earth will pass away, but my words will not pass away.
Plus, we also have to consider what the New Testament authors didn’t say. They didn’t have restore any lost books or corrupted scripture. No, when they quoted from the Old Testament, they seemed to have complete confidence that those words were as accurate as the day they were first said or written down. They trusted that God had preserved the law and the prophets’ words for 2000-3000 years. So why can’t we have the same confidence that God has preserved the New Testament for the past 2000 years?
A common theme I hear from Mormons is the idea that we should follow the model of the church shown in the New Testament. The New Testament church obviously put a lot of trust and confidence in the accuracy of scriptures from previous prophets and the apostles. Can we really follow the New Testament model of the church if we have some lingering distrust in the text of the New Testament? Just because people disagree about how to interpret certain passages in the Bible does not mean those passages have been corrupted.
Some Mormons I’ve talked to have quoted verses that mention books that aren’t in the Bible.
1 Kings 14
19 Now the rest of the acts of Jeroboam, how he warred and how he reigned, behold, they are written in the Book of the Chronicles of the Kings of Israel.
There are a few things wrong with that response. If those lost books were important, inspired scripture, then why don’t the LDS restored books include the Book of the Chronicles of the Kings of Israel?
Secondly, the New Testament authors quote from quite a few Old Testament books, but not from “lost books” like the Book of Abraham or restored text like the Book of Moses.
Finally, there were certainly other books written during the time Joseph Smith wrote the Doctrine and Covenants, but LDS church doesn’t include them as scriptures. Claiming the Bible is complete and preserved doesn’t mean it includes every book written during Biblical times.
To be clear, we’re not claiming the text of the Bible is 100% the same as it was 2000 years ago. There is evidence there have been small changes, like punctuation errors. But none of the changes impact the meaning or interpretation of any fundamental doctrine passages. The belief in a preserved Bible and Gospel means God has protected His message to us enough that it can be trusted.