Why does God send them a strong delusion?

God sends a strong delusion because they rejected the love of the truth, or the love of Christ, refusing salvation. Since they received not the truth that they might be saved, God removes their ability to comprehend truth, causing them to believe the lie, which sets the tribulation in motion.

Les explains from 2 Thessalonians chapter 2 that the Antichrist's coming is after the working of Satan, with all power, signs, and lying wonders, and with all deceivableness of unrighteousness. Those who perish are those who received not the love of the truth, or the love of Christ, that they might be saved. Because they rejected salvation, God shall send them a strong delusion: He takes away their ability to comprehend truth, and they believe the lie. That, Les says, is what sets the tribulation ball rolling, as the world again believes the lie of the Father of all lies, Satan.

2 Thessalonians 2:11

And for this cause God shall send them strong delusion, that they should believe a lie:

What will God do to their ability to understand truth?

God will send them a strong delusion and take away their ability to comprehend truth. Because they rejected the love of the truth—synonymous with Christ—and refused salvation, He will remove their understanding, and they will believe the lie, setting the tribulation in motion.