How can I trust copies of the Bible when we don't have the originals?

No ancient document has a surviving original—that's true of every text from that era, not a special problem for the Bible. The real question is how good the copies are, and how many independent ones exist to compare.

By that standard, the New Testament is unusually well documented. Julius Caesar's Gallic Wars survives in around ten manuscripts, the earliest copied about 900 years after he wrote it. The New Testament survives in thousands of Greek manuscripts, some fragments dating within a century or so of the originals—an enormous gap in both quantity and closeness compared to almost any other ancient text.

The Old Testament has its own remarkable evidence. The Dead Sea Scrolls, discovered in the 1940s, include a complete copy of Isaiah dated to roughly 125 BC—about a thousand years earlier than the previously oldest known complete Hebrew Old Testament manuscript. When scholars compared them, the texts were nearly identical, mostly differing in spelling.

None of this proves every theological claim in the Bible. But as a purely historical question—can we reconstruct with confidence what the original writers wrote?—the evidence is stronger here than for almost any other document from the ancient world.