Interesting use of the article in 1 Thessalonians 1:8-9

I had some discussion with a Jehovah's Witness earlier on this blog about the deity of Our Lord Jesus Christ. One objection that they always make is that the definite article is not used in John 1:1 where it says "and the Word was God." They want to make that mean "a god," as if the Jewish mind would even entertain that thought.

While preparing for the Sunday School lesson tomorrow, I found it interesting that in 1 Thessalonians 1:9, a very similar construct occurs, where the definite article is used before one occurrence of "God" but not another. Here is the relevant verse from the 1881 Westcott-Hort New Testament, as shown on Bible Gateway.

αυτοι γαρ περι ημων απαγγελλουσιν οποιαν εισοδον εσχομεν προς υμας και πως επεστρεψατε προς τον θεον απο των ειδωλων δουλευειν θεω ζωντι και αληθινω

Observe that the definite article appears in the phrase προς τον θεον, which is the same phrase used in John 1:1 and John 1:2. It does not occur in the phrase δουλευειν θεω ζωντι και αληθινω at the end of the verse. I am not enough of a Greek scholar to draw any definite conclusions on this. I do wonder whether it indicates nothing more than a tendency to use the article after προς, but not necessarily in all other contexts. Observe, for example, verse 1 of this chapter.

παυλος και σιλουανος και τιμοθεος τη εκκλησια θεσσαλονικεων εν θεω πατρι και κυριω ιησου χριστω χαρις υμιν και ειρηνη

Notice that there is no definite article in the phrase εν θεω πατρι, "in God (the) Father." Clearly this use without the article refers to God, not "a god." This just weakens the Arian argument that much more in my mind.