One of the more common ones is that we don't even know what the Bible originally said – it has been corrupted throughout the centuries.
People have been told (rightly) that we have none of the original manuscripts, but then people conclude (wrongly) that we can’t know what those original manuscripts said. That is simply incorrect. The huge number of manuscripts we have from various parts of the world, and the massive number of quotations of Scripture from other ancient sources, means that (as the agnostic New Testament scholar, Bart Ehrman, said), “the essential Christian beliefs are not affected by textual variants in the manuscript tradition of the New Testament”.
But let me illustrate how we aren’t at the mercy of scholarly opinion if we want to have confidence that we have what was originally written.
Suppose Uncle Edgar developed a hair-restoring elixir. He wrote down the ingredients and instructions for making the potion and left them in a drawer without telling anyone. Then he died. Uncle Edgar had no children so his six nephews were given the task of going through his stuff and deciding what to throw out and what to keep. As they were rummaging about, they were talking about how their uncle Edgar had such a good head of hair right up into old age, and then they opened the drawer and discovered his secret – they found the instructions for the elixir. They each made a copy, but as none of them were bald at that time they didn’t need to use it, so during the decades that passed, Uncle Edgar's original was lost.
When the nephews eventually started to lose their hair they remembered the instructions for the elixir, so they each got out their copies. What they discovered was that no two copies were exactly the same. It seems that in their rush, some mistakes were made. Does that mean they can never know what Uncle Edgar wrote? Well, no, it doesn’t mean that at all. Even if all the nephews made mistakes, they wouldn’t all make mistakes at the same place, and so by comparing the six imperfect copies you could get back to the original.
But if the sceptic insisted that we can’t be 100% sure that we have the right formula there is a very simple way to check – make the elixir and see if it works. In the same way, for centuries people all over the world have followed the Bible’s “instructions” for how to have new life, and the result is they get new life – they obviously were following the right instructions.
You can be sure the message of the original authors hasn't been lost or changed, because the message of the Bible is still "the power of God for salvation to everyone who believes" (Romans 1:16 ESV).