THE Angel of the Lord, or AN Angel of the Lord?

I had an interesting email regarding whether the angel of the Lord in Matt 1:20; 2:13 can be the same one in the OT:

"Matthew 1:20 I looked at other translations other then the KJV for 'Angel of the Lord' in the new testament. I looked in the KJV in the old testament and the new and it is 'The Angel of the Lord' But looking in other bible versions I see it has 'The Angel of the Lord' in the old but 'An Angel of the Lord' in the new. I believe that the new testament Angel of the Lord is not 'THE Angel of the Lord' but 'An Angel of the Lord.'"

Reply: You can see how a Bible is translated can often lead to confusion.

If you look at both the Greek text in the New Testament, and the Greek text in the Old Testament (the LXX/Septuagint) they both have the exact same construction. Both are in the genitive construction [KURIOU], both are anarthrous (they lack the definite article "The").
Ex 3:14 "AGGELOS KURIOU"
Matt 1:20 and 2:13 "AGGELOS KURIOU"
The definite article is still placed in other modern Bibles. The following still render "THE angel of the Lord" at Matt 1:20:
New American Bible
The Webster Bible
American KJV
Lattimore's NT
Jerusalem Bible
New Jerusalem Bible

Of course, none of this really means anything, as we can see by looking at the New American Standard Version at Acts 7:30:

"After forty years had passed, *an angel* appeared to him in the wilderness of Mount Sinai, in the flame of a burning thorn bush. When Moses saw it, he marveled at the sight; and as he approached to look more closely, there came the voice of the Lord: 'I am the God of your fathers, the God of Abraham and Isaac and Jacob.' Moses shook with fear and would not venture to look."

Now compare this with "THE Angel of the LORD" at Exodus 3:2, 14, 15 etc.

"One of the many theologically significant constructions is AGGELOS KURIOU. In the LXX this is the normal phrase used to translate 'the angel of the Lord.' The NT exhibits the same phenomenon, prompting Nigel Turner to suggest that AGGELOS KURIOU is not *an angel* but *the angel of the Lord.* Indeed, although most scholars treat AGGELOS KURIOU in the NT as *an angel of the Lord*, there is no linguistic basis for doing so. A part from theological argument, it is most probable that AGGELOS KURIOU is THE angel of the lord in the NT and is to be identified with THE angel of the lord of the OT." Greek Grammar Beyond The Basics by Daniel B. Wallace

We see that the New World Translation Bible handles this nicely by rendering both occurrences (OT and NT) as "Jehovah's Angel."