Frequently , people delude themselves with no help from God.
Jas 1:22 But prove yourselves doers of the word, and not merely hearers who delude themselves.
But why did the people in 2nd Thess receive delusion from God?
2 Thes 2:8; Then that lawless one will be revealed whom the Lord will slay with the breath of His mouth and bring to an end by the appearance of His coming;
2 Thes 2:9; that is, the one whose coming is in accord with the activity of Satan, with all power and signs and false wonders,
2 Thes 2:10; and with all the deception of wickedness for those who perish, because they did not receive the love of the truth so as to be saved.
2 Thes 2:11; For this reason God will send upon them a deluding influence so that they will believe what is false,
2 Thes 2:12; in order that they all may be judged who did not believe the truth, but took pleasure in wickedness.
It tells us the reason, God sends a delusion, and it specifically says who He will send it to. The "lawless one" is generally taken to be the beast of the "end-times"
or "the last days". (both phrases pertain to this age).
It isn't Christians who are deluded, it is those who refuse to "receive the love of the truth".
Matt 24:4; And Jesus answered and said to them, "See to it that no one misleads you.
Matt 24:5; "For many will come in My name, saying, 'I am the Christ,' and will mislead many.
Who... what.. are the false Christs?
There are certain many "christs" in the world today. Just in the last few years we've had people appearing claiming to be Jesus re-incarnated.
We've had people claiming to see Jesus in secret places, or out in the woods in a clandestine meeting.
Matt 24:25; "Behold, I have told you in advance.
Matt 24:26; "So if they say to you, 'Behold, He is in the wilderness,' do not go out, or, 'Behold, He is in the inner rooms,' do not believe them.
Matt 24:27; "For just as the lightning comes from the east and flashes even to the west, so will the coming of the Son of Man be.
There are "Jesus's" who aren't deity, Jesus's who who weren't sinless, Jesus's who were never resurrected, Jesus's who were resurrected as someone else,
Jesus's who aren't the only begotten Son of God, ... we even have some people still waiting for a Christ (Messiah)
None of those Jesus's are the Jesus on the Bible.
We have the Dalai Lama, Buddha, Shiva, Allah, etc... all claiming to to be the way to heaven and enlightenment.
None of them are the Christ of the Bible either... as Jesus says in Matt 24. Many will be led astray.