problem here is, a lot of parts of the Bible specially the oldest parts were passed down orally so how could you be sure that it's consistent?
Not orally. They were written down, then copied and recopied by scribes word for word. But even then, the risk of error, whether deliberate or not, is still there.
So how can you be sure?
It all depends on your faith.
If you believe that God caused the bible to be written, then you should also believe that God will also make sure that the correct version is available and can be easily found.
If you don't believe it's possible to know which version is correct, then it would be illogical to believe that God caused the bible to be written.
And if you don't believe God caused it to be written, then logically, you shouldn't bother studying the bible because it's only going to be an enormous waste of your time.
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Here's an example.
Anyone who quotes the bible by saying, "let he who has not sinned cast the first stone" is a novice in scriptural studies.
John 7:53-8:11 is a forgery that was added to the original text. It's called the "Pericope Adulterae":
http://en.wikipedia.org/wiki/Jesus_and_the_woman_taken_in_adulteryIt's true that there are unreliable portions of the bible. But if you're diligently searching for God's Word, it's not going to be too difficult to distinguish the reliable portions from the unreliable ones.