The answer
"And with all unrighteous deception among those who perish, because they did not receive the love of the truth, that they might be saved. And for this reason God will send them strong delusion that they should believe the lie that they all may be condemned who did not believe the truth but had pleasure in unrighteousness" (2 Thess 2:10-12).In verse 10, God gave the reason for allowing that to happen. God gives man the free will and the opportunity to choose between truth and sin. When man rejects the truth, he declares his rejection of God; and therefore should expect God's punishment.
God had permitted that strong delusion to influence them. The Holy Scriptures repeatedly represent God as doing what He only permits to be done. Nothing can be done in heaven, on earth, or in hell, without either his immediate power or permission. This is the reason why the Holy Scripture speaks as above.
These verses relate nicely with what God has done with Zedekiah when he refused to hear the truth from the prophet Micaiah.
Answered by His Eminence Metropolitan Youssef (opens in a new tab)