Here is why it’s hard to alter the text of the Bible.
The page at right is the first chapter of Hebrews in the ancient Bible manuscript called Codex Vaticanus.
At Hebrews 1:3 is a mistake: In place of “upholding [pheron] all things,” the scribe has written “revealed [phaneron] all things.”
But a later user of this Bible – who of course has this verse properly memorized – has written in the margin:
ἀμαθέστατε καὶ κακέ, ἁφες τὸν παλαιόν, μὴ μεταποἰει!
"Unlearned and evil! Leave the old, do not change it!"
If anyone wanted to change the text of scripture, which was already diffusing throughout the Mediterranean and Near Eastern world by the end of the fist century – he would have to convince hundreds of independent communities to all change the same word in every single one of their manuscripts and to intentionally forget all that they had previously memorized.
But in fact, we know all the variations that have popped up in the Greek Bible manuscripts before the invention of the printing press – they're all undramatically shown in the footnotes of your Greek Bible – and practically every variant is either a spelling error or an obvious mistake like the one above. The Church’s immune system against changes to scripture lies partly in Christian people who already know what the text says, and partly in our enormous bookshelf of service books that contain what scholars call “The Byzantine Text.”
Incidentally, Codex Vaticanus has been scanned and posted on the internet – read it online! See what the Bible looked like in the fourth century. (Spoiler: It’s the same as ours today. Only in Greek. And in fantastic penmanship.)