The apostle Paul did not specifically state who the restrainer is, but this can be ascertained from both his words at 2 Thessalonians 2 and Jesus illustration at Matthew 13. So why didn't Paul just come out and say it as the original post asks ?
Because the Bible is written in a very specific way, as Jesus said in prayer to the Father: "I publicly praise you, Father, Lord of heaven and earth, because you have hidden these things ("sacred secrets of the Kingdom" or "mysteries of the kingdom", Matt 13:11) from the wise and intellectual ones and have revealed them to young children. Yes, O Father, because this is the way you approved".(Matt 11:25, 26)
The Bible is a "closed book" to most, for Jehovah God inspired it in such a way that most cannot grasp its teachings, unable to assemble it like a picture puzzle. Jesus, in speaking about the "sacred secrets of the Kingdom" through a series of illustrations, used the Greek word syniemi some 6 times, that literally means to "mentally put the pieces together".(Matt 13: 13, 14, 15, 19, 23, 51)
Only "young children" ("babes", KJV) or those who allow themselves to be taught by Jehovah God will ever be able to unlock what the Bible really teaches, including what the apostle Paul wrote to the Thessalonians concerning apostasy.
Because most who profess to be Christian reject or set aside the one true God, Jehovah, then what Jesus said could not be fulfilled in them. Concerning the "woman" at Isaiah 54, Jesus, quoting from verse 13, said that "It is written in the Prophets: ' They (her "sons") will all be taught by Jehovah.' Everyone who has listened to the Father (Jehovah God) and learned comes to me".(John 6:45)
Thus, only those who are "taught by Jehovah" are able to understand the content and meaning of the Bible, which means grasping what Paul meant by the one who "is acting as a restraint" to the apostasy that began proliferating after the apostles died.
At Proverbs 25, it says that "it is the glory of God to keep a matter secret, and the glory of kings is to search through a matter. As the heavens are high and the earth is deep, so the heart of kings is unsearchable."(Prov 25:2, 3) God reveals the understanding of his written word through these "kings", those anointed with holy spirit for the "heavenly calling".(Heb 3:1) Jesus was more specific when he said that during our time frame, that he appointed "a faithful and discreet slave" to give members of the "household....meat in due season" (Matt 24:45), unlocking the mysteries of the Bible.
Paul said told the Ephesian older men in about 56 C.E., that "I know that after my going away oppressive wolves will enter in among you and will not treat the flock with tenderness, and from among you yourselves men will rise and speak twisted things to draw away the disciples after themselves".(Acts 20:29, 30)
The apostolic influence that acted as a restraint against this foretold apostasy, continued until the death of John in about 100 C.E. The rapid influx of apostate ideas and false doctrines and practices began to gain a serious foothold even before John's death, for in writing his three letters, said: "Young children, it is the last hour (before apostasy took firm hold), and just as have heard (John had been warning them that it was coming) that the antichrists (those who have twisted who Jesus really is) is coming, even now many antichrists have appeared, from which fact that it is the last hour (that a firm stand can be taken by the apostles, before apostasy begins to flourish, teaching false religious doctrines and practices)".
So, after John died, none of the so-called apostolic successors (such as the "Apostolic Fathers" or early "Church Fathers") showed any of the restraining influence and strength of the apostles. Rather, many contributed to the foretold apostasy.
For example, contrary to the practice instituted by Jesus at the Lord’s Evening Meal, known also as the Last Supper, the author of The Didache (or Teaching of the Twelve Apostles in which some historians date it before or about the year 100 C.E. Its author is unknown) advised the passing of the wine before the bread. (Matthew 26:26, 27)
This writer also stated that if no body of water was available to perform baptism by immersion, pouring water on the head of the baptism candidate would suffice, though Jesus showed that a person was to be totally "dipped under" the water.(Mark 1:9, 10; Acts 8:36, 38) The same text encouraged Christians to observe such rituals as obligatory fasting twice a week and recitation of the Our Father exactly three times a day, which Jesus did not establish.(Matthew 6:5-13; Luke 18:12)
For his part, Ignatius (who was alive during the time of the apostle John) envisioned a new organization of the Christian congregation (or "church"), with just one bishop presiding “in the place of God.”(The Anti-Nicene Fathers: Translations of the Writings of the Fathers down to Down to A. D. 325, pg 61, 1885)
This bishop would hold authority over many priests, which eventually lead to Catholicism and other religions. Such inventions opened the way for further waves of unscriptural teaching, contrary to Jesus saying that all were to be "brothers".(Matthew 23:8, 9)
Exaggeration carried some Apostolic Fathers adrift. Papias thirsted for truth and referred to the Christian Greek Scriptures (commonly called the New Testament). At the same time, he believed that during the foretold Thousand Year Reign of Christ, grape vines will produce 10,000 branches, each branch 10,000 twigs, each twig 10,000 shoots, each shoot 10,000 clusters, each cluster 10,000 grapes, and each grape the equivalent of 1,000 quarts of wine.(A Dictionary of Christian Biography, Literature, Sects and Doctrines, pg 189, 1887)
Polycarp was willing to die a martyr’s death rather than renounce his Christian faith. It is reported that he was instructed by the apostles and others who knew Jesus. He quoted from the Bible, and it appears that he strove to live by Christian principles. The devotion that some had to Polycarp, however, verged on idolatry. The Martyrdom of Polycarp states that after his death, the “faithful” were eager to claim his remains. They considered his bones “more precious than the most exquisite jewels, and more purified than gold.” (Apostolic Fathers, by Kisopp Lake, 1912, chp 18) Clearly, the poisoned waters of apostasy were surging.
Some Apostolic Fathers accepted extra-Biblical texts as if they were inspired. Clement of Rome (died 101 C.E.?), for one, cites the apocryphal works Wisdom (of Solomon) and Judith. (The book is thought to have been written in Palestine during the Greek period toward the end of the second century or the start of the first century B.C.E.) The writer of The Epistle of Polycarp refers to Tobit (estimated to be of about the third century B.C.E.) to give credence to the idea that the giving of alms has power to deliver the giver from death.
And these are just "tip of the iceberg" concerning apostate ideas that began to emerge just before and after the death of the apostles, who held such twisted thinking at bay. The apostle Paul wrote to Titus: "But have nothing to do with foolish arguments and genealogies and disputes (about what someone's personal view the Bible says) and fights over the (Mosaic) Law, for they are unprofitable and futile. As for a man who promotes a sect (or one who promotes something different than what the apostles taught), reject him after a first and second admonition, know that such a man has deviated from the way (or "one faith", Eph 4:5) and is sinning and is self-condemned."(Titus 3:9-11)
