The torah is pretty much the same as the old testament; that and all or most of the new testament were originally written in Hebrew. The whole lot was later translated into Greek and eventually into various other languages.

After the natural degrading of paper and parchment over time, and the fanatical book burnings of Rome, Judiasm, and Islam, there are still a remarkable number of fragments of multi-thousand year old texts in existance. These together with the science of textual analysis mean that we have a very good idea of what the originals contained.