How can we prepare ourselves for the Antichrist's deception that Paul writes about in 2 Thessalonians 2:9-12?
The coming of the lawless one by the activity of Satan will be with all power and with pretended signs and wonders, and with all wicked deception for those who are to perish, because they refused to love the truth and so be saved. Therefore God sends upon them a strong delusion, to make them believe what is false, so that all may be condemned who did not believe the truth but had pleasure in unrighteousness (2 Thess. 2:9-12).
It's true that Paul speaks of a great deception that will befall those who are to perish when the “lawless one” (or Antichrist) comes. Even the Catechism affirms in paragraph 675 that the faith of many believers will be shaken in the final trial before the Second Coming and that many will fall prey to “a religious deception offering men an apparent solution to their problems at the price of apostasy from the truth.”
The key to protecting ourselves from such deception, which is present even in the world today (1 John 4:3), is found in 2 Thessalonians 2:15: “Stand firm and hold to the traditions which you were taught by us, either by word of mouth or by letter.” For Paul, the word of God that is handed down to us by the Church, whether in its written form or unwritten form, is our source of protection.
Having a firm grasp of the definitive teachings of the Church will empower us to discern what is true and what is false. As St. John writes, “Whoever knows God listens to us, and he who is not of God does not listen to us. By this we know the spirit of truth and the spirit of error” (1 John 4:6).
For more details on this subject, you can read my article "The Secret to Protecting Yourself from the Antichrist's Deception."