No, the first two letters in Greek are χλ. (If I’m missing a joke, my apologies.)
What I meant to say is that the consonantal diagraph, while two letters in our alphabet, transliterates one letter in Greek. Spanish by the way, also has a C and a CH, though the sound there is soft as in chocolate).
It’s interesting that the decision to transliterate the χ as “ch” is not consistent. The Magna Carta, for example, could have been Magna Charta, though Latin seems to be the culprit there, and cartography could be chartography (the OED lists that as an alternative spelling but only has one citation for that). Archbishop could be pronounced arkbishop, though, once again Latin or more specifically Italian, seems to have softened the diagraph, but not, for some reason, in the word archangel.
The ch is also an independent letter in Welsh, Irish and Scottish (Loch Lomond and Loch Ree come to mind. Wiki suggests that the Irish letter C with a dot over it was changed to ch when typewriters were invented. I’m not sure about that...).
Perhaps Dave could have said that the symbol Cl comes from the first two sounds of the word “chlorine.” Or. more academically, he could have said, first two phonemes .... Chlorine could have been rendered Clorine.
In any case, mine was but an idle post but lh’s response led me through a marvelous walkabout in the OED.