In 2 Thessalonians 2.11 we are told that in the end times the people of the Earth fall for a lie.
For the mystery of iniquity doth already work: only he who now letteth will let, until he be taken out of the way. And then shall that Wicked be revealed, whom the Lord shall consume with the spirit of his mouth, and shall destroy with the brightness of his coming: Even him, whose coming is after the working of Satan with all power and signs and lying wonders, And with all deceivableness of unrighteousness in them that perish; because they received not the love of the truth, that they might be saved. And for this cause God shall send them strong delusion, that they should believe a lie: That they all might be damned who believed not the truth, but had pleasure in unrighteousness. 2 Thessalonians 2.7-12
We are given some major hints, first off this lie originates with the "Mystery of Iniquity" and people who do not love the truth fall for the lie.
The "Mystery of Iniquity" is made up of the "workers of iniquity" in Matthew chapter 7.15-23, they are the "fruit" of the "many false Prophets" The LORD Jesus warned us were coming.
This word "iniquity" means "no law".
These people claim to serve Jesus, but they do not work God's Law in their lives.
These people are absolutely convinced they are Christians, but Jesus proclaims He does not know them.
They refuse to observe God' Seventh Day Sabbath which is the eternal / perpetual SIGN of who serves Him.
They lack the SIGN of belonging to The LORD so He does not acknowledge them.
They do not love God's word, instead they obey The Pope who commands that the Seventh Day be desecrated.
Whoever you obey that is your master:
Know ye not, that to whom ye yield yourselves servants to obey, his servants ye are to whom ye obey; whether of sin unto death, or of obedience unto righteousness? Romans 6.16
The Pope commands that sin be committed by desecrating The Seventh Day in Cannon Law #29.
Christians must not judaize by resting on the Sabbath, but must work on that day, rather honoring the Lord's Day; and, if they can, resting then as Christians. But if any shall be found to be judaizers, let them be anathema from Christ.
The LORD commands that The Seventh Day be kept Holy forever.
Whoever you obey is your master.
It is the lack of love for God's word which is truth leads them to believe "The Lie".
So what is truth??
Jesus told us that God's word is truth:
Sanctify them through thy truth: thy word is truth.John 17.17
These people have rejected God's word, the truth, for the lies of Mystery Babylon.
So what is "The Lie"??
The lie is that any Sunday / Xmas keeper is a Christian at all.