Letters as in epistles or letters as in letters of the alphabet?

I'm guessing as opposed to systems where each sign is considered to be an entire word or idea, like Sumerian or Chinese. Even then, they often had systems where certain signs would stand for a sound, so that they could write foreign words like names. So these could be called "letters."

The earliest all-letters-no-ideograms system we have is the Phoenician alphabet, c.1200 BC, which is the type called an abjad: it has signs only for consonants. this works for Semitic languages. The Greeks added vowels in the 8th C BC.