The Westcott-Hort critical version is superior to Textus Receptus. The critical versions (SBL, NA, WH etc) are easily trusted for their textual reliability, on which the New or Revise English versions are based. The article is absent with God, however, that doesn't mean it should be interpreted as "a god" to denote pagan gods.
John MacArthur equates God with Satan, and interprets 1Cor 14 basically as sarcasm. Here is the quote on his study Bible NASB 1Cor14:2,
NASB For one who speaks in a tongue does not speak to men but to God; for no one understands, but in [his] spirit he speaks mysteries.
‡ 14:2 one who speaks in a tongue. This is singular (see previous note; cf. vv. 4, 13, 14, 19, 27), indicating that it refers to the false gibberish of the counterfeit pagan ecstatic speech. The singular is used because gibberish can’t be plural; there are not various kinds of non-language. There are, however, various languages; hence when speaking of the true gift of language, Paul uses the plural to make the distinction (vv. 6, 18, 22, 23, 29). The only exception is in vv. 13, 27, 28 (see note there), where it refers to a single person speaking a single genuine language.
does not speak to men but to God. This is better translated, “to a god.” The Gr. text has no definite article (see similar translation in Ac 17:23, “an unknown god”). Their gibberish was worship of pagan deities. The Bible records no incident of any believer ever speaking to God in any other than normal human language.
no one understands … in his spirit he speaks mysteries. The fleshly, or carnal, Corinthians using the counterfeit ecstatic speech of paganism were not interested in being understood, but in making a dramatic display. The spirit by which they spoke was not the Holy Spirit, but their own human spirit or some demon; and the mysteries they declared were the type associated with the pagan mystery religions, which was espoused to be the depths that only the initiated few were privileged to know and understand. Those mysteries were totally unlike the ones mentioned in Scripture (e.g., Mt 13:11; Eph 3:9), which are divine revelations of truths previously hidden (see notes on 12:7; Eph 3:4–6).
He doesn't understand how Greek article works. The use of the article may definitise a noun, but lack of it doesn't necessarily make it indefinite. Robert Young writes, “When the author wants to focus on the quality, character, nature, or class of the noun, he will omit the article” (68). Likewise, Zerwick notes, “The omission of the article shows that the speaker regards the person or thing not so much as this or that person or thing, but rather as such a person or thing, i.e., regards not the individual but rather its nature or quality” (Biblical Greek, 55)”.
It is clear from the context, the focus of is not on the nouns but on the action of speaking; here, both God and Spirit are stated in parallel, it is very likely that both God and Spirit would be anarthrous (lacks the article). They cannot be interpreted as indefinite. He avoids calling the Spirit evil, by following the NASB translation of treating the Spirit as his own spirit. It is correct to treat Pneuma as his personal Spirit, as v14-16 also explicitly says "my spirit, my mind", indicating that the man who speaks in unknown tongues speaks spiritually, within himself, not to others.
If searched the string on theword, we find there are at least 12 verses where God occurs without any article in the verse in first Corinthians.
NASB 1Cor 12:3 Therefore I make known to you that no one speaking by the Spirit of God says, "Jesus is accursed"; and no one can say, "Jesus is Lord," except by the Holy Spirit.
There is no article with any noun here, I doubt MacArthur would claim these are indefinite nouns. In 1Cor 15:10 "But by the grace of God I am what I am" also has God without article (χάριτι δὲ θεοῦ εἰμι ὅ εἰμι), where the focus on grace, not on God.
On verse 14:14–17, he says:
Paul continued to speak sarcastically (cf. v. 16; 4:8–10) about counterfeit tongues, so he used the singular “tongue” (see note on vv. 2–39), which refers to the fake gift. He was speaking hypothetically to illustrate the foolishness and pointlessness of speaking in ecstatic gibberish. The speaker could not understand, and what virtue is there in praying to God or praising God without understanding? No one can “Amen” such nonsense.
14:18 I speak in tongues more than you all. Paul emphasized that by writing all of this, he was not condemning genuine tongues (plural); nor, as some may have thought to accuse him, was he envious of a gift he did not possess. At that point, he stopped speaking hypothetically about counterfeit tongue-speaking. He actually had more occasions to use the true gift than all of them (though we have no record of a specific instance). He knew the true gift and had used it properly. It is interesting, however, that the NT makes no mention of Paul’s actually exercising that gift. Nor does Paul in his own writings make mention of a specific use of it by any Christian.
On the reference of Paul speaking in tongues, MacArthur turns separates it as sarcastic or genuine at his own convenience. It is clear that his interpretation is not accurate. The point about the adversative conjunction "but" in the verse, by Alesja Lavrinovica is also very strong to refute the parody interpretation of MaCarthur.
There is another error in his commentary about the definite article. On Luke 6:20 note:
you who are poor. Christ’s concern for the poor and outcasts is one of Luke’s favorite themes (see Introduction: Historical and Theological Themes). Luke used a personal pronoun (“you”) where Matt. 5:3 employed a definite article (“the”); Luke was underscoring the tender, personal sense of Christ’s words
Luke 6 also has the definite article with the substantives (Μακάριοι οἱ πτωχοί) like in Matthew, the only difference is "yours" instead of "theirs".
I also disagree with his comment on John 3:10 ὁ διδάσκαλος τοῦ Ἰσραὴλ "the teacher of Israel" where he writes, "The use of the definite article “the” indicates that Nicodemus was a renowned master-teacher in the nation of Israel, an established religious authority par excellence". There is dispute among scholars whether this should be translated as "the teacher" as some reputed one, or as indefinite. I think the new versions (RV, ESV, NET) are mistaken for using "the teacher", compared to the KJV, RSV, CSB etc (indefinite: a teacher).
The presence of article means nothing, as the frequency of article is a lot higher in Greek; it is used mainly for emphasis and stylistic purpose. We cannot interpret Greek articular phrases the same as in English to be simply definite or indefinite; as we know there is no indefinite article in Greek, and its definite article doesn't even mean and called definite article.