Is the article likely original to the letter?
We cannot know.
The verse appears with the definite article ὁ (appearing in the verse in the dative case, τῷ) in the majority of manuscripts, as well as in one later copy of the Codex Sinaiticus (of which the base copy dates to the 4th century), in a later copy of the Bezae Codex (dating back to the 5th century), and some later manuscripts.1
The verse appears without the definite article ὁ in the Codex Vaticanus (4th century) and, perhaps most importantly, in the Papyrus P46, which dates to the late 2nd/early 3rd century.2
It is impossible, however, to know what was in the "original". The later manuscripts may have derived from an earlier lost manuscript.
Is it possible that MacArthur is correct and it should be "a god" instead of "God"?
The phrase in question was quoted by both John Chrysostom and by Ambrose in antiquity. Both understood the passage to refer to God and not "a god" in their writings - Chrysostom in his Homily XXXV on 1 Corinthians and Ambrose in his treatise On the Holy Spirit (II.XII.131). Chrysostom is probably the more interesting witness of the two, as John MacArthur himself has called Chrysostom "perhaps the greatest preacher of the early Church."3
Chrysostom's quote of 1 Corinthians 14:2 omits the definite article4:
ὁ λαλῶν γλώσσαις, οὐκ ἀνθρώποις λαλεῖ, ἀλλὰ Θεῷ·
It is clear from the context of his homily, however, that it was understood that this meant "God" and not "a god". In no where in his explanation does he indicate that speaking in tongues was useless because one was speaking in vain to some "god":
At this point he makes a comparison between the gifts, and lowers that
of the tongues, showing it to be neither altogether useless, nor very
profitable by itself. For in fact they were greatly puffed up on
account of this, because the gift was considered to be a great one.
And it was thought great because the Apostles received it first, and
with so great display; it was not however therefore to be esteemed
above all the others. Wherefore then did the Apostles receive it
before the rest? Because they were to go abroad every where. And as in
the time of building the tower the one tongue was divided into many;
so then the many tongues frequently met in one man, and the same
person used to discourse both in the Persian, and the Roman, and the
Indian, and many other tongues, the Spirit sounding within him: and
the gift was called the gift of tongues because he could all at once
speak divers languages. See accordingly how he both depresses and
elevates it. Thus, by saying, He that speaketh with tongues, speaketh
not unto men, but unto God, for no man understandeth, he depressed
it, implying that the profit of it was not great; but by adding, “but
in the Spirit he speaketh mysteries” he again elevated it, that it
might not seem to be superfluous and useless and given in vain.5
Furthermore, there are countless occasions in the New Testament where the definite article is omitted and it is clear that the text is speaking of God. Examples:
ὁ δὲ ἀποκριθεὶς εἶπεν· γέγραπται· οὐκ ἐπʼ ἄρτῳ μόνῳ ζήσεται ὁ
ἄνθρωπος, ἀλλʼ ἐπὶ παντὶ ῥήματι ἐκπορευομένῳ διὰ στόματος θεοῦ.
Jesus answered, “It is written: ‘Man shall not live on bread alone, but on every word that comes from the mouth of God’” (Matthew 4:4)
Οὐδεὶς δύναται δυσὶ κυρίοις δουλεύειν· ἢ γὰρ τὸν ἕνα μισήσει καὶ τὸν
ἕτερον ἀγαπήσει, ἢ ἑνὸς ἀνθέξεται καὶ τοῦ ἑτέρου καταφρονήσει. οὐ
δύνασθε θεῷ δουλεύειν καὶ μαμωνᾷ.
“No one can serve two masters. Either you will hate the one and love the other, or you will be devoted to the one and despise the other.
You cannot serve both God and money" (Matthew 6:24)
πᾶσιν τοῖς οὖσιν ἐν Ῥώμῃ ἀγαπητοῖς θεοῦ, κλητοῖς ἁγίοις, χάρις ὑμῖν
καὶ εἰρήνη ἀπὸ θεοῦ πατρὸς ἡμῶν καὶ κυρίου Ἰησοῦ Χριστοῦ.
To all in Rome who are loved by God and called to be his holy people: Grace and peace to you from God our Father and from the Lord Jesus
Christ (Romans 1:7)
In the above I have cited the NIV translation of the Greek, which is one of the translations used in the MacArthur Study Bibles.
1. Apparatus, Nestle-Aland Greek-English New Testament
3. MacArthur New Testament Commentary
4. Patrologia Graeca 61, p.296
5. Homily XXXV on 1 Corinthians, No. 1