if i learn to speak Greek fluently, could i possibly come to understand ancient text? the Bible? furthermore, to what extent to they differ?

No. Just as English has changed over the centuries, modern and ancient greek have changed as well. and parts of the biblical texts were written in other than Greek. Which was translated to Latin. Which was translated to English. which was modernized several times.

Only the New Testament was originally written in a type of Greek called "Koine Greek" (sometimes called "Hellenistic Greek" or "New Testament Greek"). It was the common lingua franca of much of the Mediterranean region and the Middle East between about 300 BCE and 300 CE. Somebody who speaks Modern Greek can still understand most of it, as the Greek language evolved less drastically than some other languages.

The Old Testament was originally written mainly in Biblical Hebrew, with some passages (Ezra 4:8–6:18 and 7:12–26, Jeremiah 10:11, Daniel 2:4–7:28) in Biblical Aramaic.

See links below for some information about the differences between ancient Greek and modern Greek.

Parts of the Bible (the Hebrew Scriptures) were originally written in Hebrew rather than Greek.