CLAIM: Paul writes, “God will send upon them a deluding influence so that they will believe what is false…” (2 Thess. 2:11). Why would God intentionally give these people a “deluding influence,” so that they wouldn’t believe in him?
RESPONSE: The key to understanding this verse is the context. God gave them a deluding influence, because “they did not receive the love of the truth so as to be saved” (v.10). In other words, this “deluding influence” only worked for those who had already denied the truth. In the following verse, it states that they were judged, because they “did not believe the truth, but took pleasure in wickedness” (v.12). In other words, the reason they believed this deluding influence was because they were evil and took pleasure in wickedness. Even Calvinist Robert Thomas states that the reason for their judgment is their willful rejection of the truth: “Their blindness will be self-imposed because of a prior refusal to ‘love the truth and so be saved.’ …The right choice could have brought them salvation and deliverance from the lawless one’s devices, but they elected not to receive God’s salvation.”
What exactly is this “deluding influence” (v.11)? In context, the deluding influence is not an inner working of the Holy Spirit to deceive people from coming to Christ. In context, the “deluding influence” is the Antichrist (v.3) and his “signs and false wonders” (v.9). If people believe that this demon possessed (v.9) and horribly wicked man is actually God incarnate (v.4), then they will have fallen prey to the “deluding influence.” Those who believe in Jesus will be impervious to this deceiver.
The active verb is used (“God will send upon them…”) because this is a form of divine judgment for rejecting the truth.
 Thomas, R. L. (1981). 2 Thessalonians. In F. E. Gaebelein (Ed.), The Expositor’s Bible Commentary: Ephesians through Philemon (Vol. 11, pp. 326–327). Grand Rapids, MI: Zondervan Publishing House.