Can you explain what Paul means by tongues being a "gift of speaking mysteries"? Does this mean that tongues are always intended to be a mysterious, unknowable "spiritual" language?
First, let’s review where Paul says in 1Corinthians 14 that speaking in tongues is a gift of speaking mysteries:
Notice Paul is writing from the perspective of the one doing the speaking. Paul says that from the speaker’s perspective, his own speech is unknowable and a mystery. This is true because when a person speaks in tongues under the influence of the Holy Spirit, he will speak in a normal, understandable human language, though it is a language the speaker himself does not understand. That is the essence of the miracle of speaking in tongues.
Remember, the gift tongues is a miracle (i.e., a work done by the Spirit) which lies beyond the ability of a person’s natural ability. Paul says "no one" will understand the speech, meaning that not even the one who speaks in tongues will understand his or her speech. Therefore, to the speaker himself the language of tongues is unknowable and a mystery, but this does not mean that tongues consists of babbling sounds of a mysterious origin.
On the contrary, "tongue" is simply the Bible's word for a foreign language. If a tongue consisted of mysterious, unknowable sounds rather than normal human language as some suggest, then what do we conclude about the events of Pentecost that occurred in Acts 2? As we demonstrated in our article, the tongues spoken in that moment were normal foreign languages as evidenced by the way foreigners in the crowd understood the speech perfectly (see Acts 2:8-11).
Furthermore, that moment in Acts 2 was the beginning of the gift of speaking in tongues in the Church. Therefore, we must define "tongues" according to the example set at that moment. Since the tongues of Acts 2 were normal human language, we must expect the gift of tongues to always include normal human languages, not babbling nonsense. The only reason for someone to propose a new definition of tongues as a “mysterious and unknowable” language would be a pretext for permitting the mindless babbling that falsely poses as the gift of tongues in so many places today.
Ironically, the Bible requires that tongues must be knowable or else it must be silenced in the gathering. Paul required that an interpreter always is present when the gift of tongues was manifested in the gathering or else the speech was to be silenced in the body. If words "tongues" meant a language that literally “no one” can understand, then how could Paul require an interpreter be present as he does in 1Corinthians 14:27? Obviously, Paul expected the language to be understandable to at least someone in the room since this was his pre-requisite for the use of tongues.
The spiritual gift of interpretation is the complementary gift God assigns to ensure the content of any foreign language share in the gathering is still understandable by the audience. In addition to the gift of interpretation, a person who naturally knows the foreign language being spoken would also be able to understand and explain it to everyone (as demonstrated in the moment of Pentecost from Acts 2).
Therefore, we must conclude that the activity that often poses as tongues today is not the true gift of tongues but is merely a modern, learned behavior with no historical or biblical precedent. False teachers have concocted this false teaching of a "mysterious" spiritual language by twisting Paul's teaching as a cover for their unbiblical practice of encouraging babbling nonsense speech. Repetitive sounds have never been considered a tongue in scripture, neither in the spiritual realm nor the human realm. In fact, Paul calls such practice ungodly:
Finally, here are some simple biblical tests you can apply to know if your practice (or anyone else’s practice) of tongues is, in fact, a gift from the Spirit and according to the Lord’s will or merely a product of human flesh:
1. Could someone mimic the exact sounds you are making? If so, it’s not the gift of tongues. It is merely a learned behavior, and when you engage in this so-called “gift,” does the entire room make similar sounds with you in unison? If so, then this is not a work of the spirit but flesh, for God alone appoints gifts and His word declares that not everyone in the body will receive the same gift according to Paul:
2. Does your speech convey a message in normal human terms (i.e., a language with syntax, vocabulary, sentence structure)? If not, it’s just babbling nonsense and not a product of the Spirit, for the Lord is not a Lord of confusion.
Once more, if your babbling can be mimicked (and all babbling can be “taught” and mimicked), then it cannot be a spiritual gift. Spiritual gifts cannot be taught nor learned, since they are always a self-evident display of God’s power, never a demonstration of man’s flesh.
3. Finally, is your speaking in tongues always accompanied by sensible interpretation as Paul required? Is such speech silenced when no interpretation is present? Are interpretations confirmed by other means to ensure they are not false themselves? Are speakers limited to only 2 or 3 speaking per gathering as Paul commanded:
If Paul's instructions are not followed, then you can know that the practice is not by scripture or the Spirit, and therefore, the activity is self-evidently not of God because the Spirit will not lead a believer into behaviors that contradict God’s word. Therefore, these behaviors must be the product of the enemy or the flesh.