Gnostic Apostasy is the key that links 2Th2:3 to 1Jn2:18.
Gnostics believe salvation comes from the internal self-realization (gnosis or knowing) of being God.
"Knowing oneself is having achieved identification with one’s own divine Being."
That self deification/divinity is consistent with the deification of the Man of Lawlessness and Son of Destruction in 2Th2:3-4. They are in Christs body (His Temple: 2 Corinthians 6:16) teaching/professing from the seat of the congregation that they have the internal realization that they are divine.
Paul warned of these imposters in similar language as John spoke in his first epistle:
Acts 20:28-30 New International Version (NIV)
28 Keep watch over yourselves and all the flock...29 I know that after I leave (Paul is the restrainer of these antichrists; 2Th2:6), savage wolves (hating the brethren; 1Jn2:9;3:15) will come in among you and will not spare the flock. 30 Even from your own number (went out from us; 1Jn2:19) men will arise and distort the truth (liars/deceivers) in order to draw away disciples after them (deceive and seduce into apostasy; 1Jn2:26).
Material reality and existence is evil according to the Gnostics;
"Generally, Gnostic cosmogony presents a distinction between a supreme, transcendent God and a blind, evil demiurge responsible for
creating the material universe, thereby trapping the divine spark
"The mortal body belonged to the world of inferior, worldly powers (the archons), and only the spirit or soul could be saved."
"This Secret Enemy resides in the lower section of the spine, and its atoms oppose the student's attempts to unite himself to his Innermost.
The Secret Enemy has so much power in the atmosphere of this world
that they can limit our thoughts and imprison our minds. When we
strive to hold the mind to one thing it will immediately attempt to
disintegrate it...These atoms evoke all that is evil within us..."
therefore, Jesus is only spirit and could not have come in the flesh, contrary to the fact Jesus himself said he was flesh.
Luke 24:39 New International Version (NIV)
39 Look at my hands and my feet. It is I myself! Touch me and see; a ghost does not have flesh and bones, as you see I have.”
John provided a test (1Jn2:4) to reveal these antichrists by asking them if they believed Jesus Christ had come in the flesh. Thus, it is clear that John was referring to the Gnostics as these antichrist apostates.
"You have heard it said antichrist is coming," John said. The only epistle bearing witness to some then current oral tradition about antichrist that suggests anything close to this is 2Th2:3 and Ac20:28-30 both from the lips of Paul to the Churches. (In fact, 2Th2:5 shows Paul was orally warning people about antichrist before he wrote about it.)
Paul was also dealing specifically with the Gnostics before his departure (death). "Knowledge" in the following scripture is a translation from Greek "gnosis" meaning "knowing". Paul privately mocked these false teachers before Timothy with a play on words, saying they "know" nothing. This scripture also indicates the apostasy was already in some motion while Paul was yet alive, even as his presence continued to restrain their influence.
1 Timothy 6:20-21 New International Version (NIV)
20 Timothy, guard what has been entrusted to your care. Turn away from godless chatter and the opposing ideas (2Th2:4--"He will oppose...") of what is falsely called knowledge (gnosis), 21 which some have professed and in so doing have departed from the faith.
First, Paul warns this apostasy (went out from us; 1Jn2:19) is coming and it reveals (manifests) who the Man of Lawlessness is, the Son of Destruction. John reminds them they have heard this before. Then he says "they went out from us (the first apostasy; 2Th2:3) in order to make manifest (reveal;2Th2:3) they were not of us". It's the same language.
Lawlessness is not "abiding in truth", "keeping Christ's commandments". God says one thing, they say another. The Pharisees nullified God's commandments by setting up their own traditions. (Mt 15:4-9) Thus by contradicting God's word and commandment, they were lawless.
This is exactly what John identified these Gnostic antichrists of doing, nullifying Christ's teaching, making them, too, lawless and thus the Man of Lawlessness and Son of Destruction. Because if His words and commands are not laid as a foundation, that house will be destroyed in the storm of judgment--the brightness of His coming. (Matthew 7:23-27; 2Th2:8)
1 John 2:4-5 New International Version (NIV)
4 Whoever says, “I know him,” but does not do what he commands is a liar, and the truth is not in that person. 5 But if anyone obeys his word, love for God is truly made complete in them. This is how we know we are in him:
The entire epistle of 1 John is written about these antichrists. Therefore, 1Jn2:4-5 is describing these antichrists in similar language of lawlessness.
But some would say "Man of Lawlessness" in 2Th2:3 is singular and refers to a singular "THE Antichrist", some evil world dictator yet future. Paul wrote:
2 Timothy 3:16-17 King James Version (KJV)
16 All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness: 17 That the MAN OF GOD may be perfect, thoroughly furnished unto all good works.
Now let me ask these persons, is there only one "man of God"? Neither is there only one "Man of Lawlessness". The "many antichrists" of the apostasy IS the "Man of Lawlessness", just as the many brethren John wrote to in his epistles is the "man of God" Paul referred to in 2Tim3:17.
"Son of Destruction" is further elaborated by Peter which follows the same pattern described by Paul and John:
2 Peter 2:1-3 New International Version (NIV)
2 But there were also false prophets among the people (Mt 15:4-9; Jews), just as there will be FALSE TEACHERS AMONG YOU (Christians; 1Jn2:19). They will secretly introduce destructive heresies, even denying the sovereign Lord who bought them (Gnostics; 1Jn2:22)—bringing swift DESTRUCTION on themselves (Son of Destruction; 2Th2:3).
Perhaps if John referenced 2Th2:3 in 1Jn2:18, saying "you have heard Paul say regarding the apostasy"... we could rest conclusively that these verses are connected with each other. The preponderance of the contextual evidence, however, still points 1Jn2:18 to 2Th2:3. John, Peter and Paul are all describing the same group of false teachers. Although John never used the name "Gnostics" in his epistles to describe these antichrist apostates, we know from the descriptions and test question he provides them, they fit the bill. Biblical context and language affords us the same confidence linking 2Th2:3-4 to 1Jn2:18-19.
Learn the lesson of Peter. In Peter's day, the Jews had a cherished eschatology. Messiah would come as a conquering warrior king like David and subjugate Rome to Jewish rule. But Jesus was crucified, proving he wasn't that Messiah. That was the scheme of the schemers in the Sanhedrin. Jesus wouldn't fit the expectation if he were crucified by Rome. Jesus=Loser; Messiah=Conqueror; therefore, Jesus is not the Messiah. The expectation makes it extremely difficult for a Jew to identify Jesus as Messiah.
Thus, Paul wrote:
1 Corinthians 1:23 New International Version (NIV)
23 but we preach Christ crucified: a stumbling block to Jews and foolishness to Gentiles,
Peter was confused by this expectation resulting in his zeal to prevent Christ's crucifixion, so driven that Jesus rebuked Satan speaking through Peter and had to reattach and heal the ear of an arresting Temple guard Peter snipped with a sword. The wrong eschatology blinds men to the teaching and purpose of God.
The same sort of thing prevents people from seeing 'antichrist' is not some world dictator but rather imposter/goat Christians who wrangle the word of God to their own destruction (as John plainly defined 'antichrist' as). Many Christians today will disassociate 2Th2:3-4 from 1Jn2:18-19 because their cherished eschatological tradition has blinded them to the possibility these verses are talking about the same people. And that is how the traditions of men nullify the word of God.