When the people turn from loving the truth and God their Creator. When people organize themselve in philosophical and political rejection of God. The natural result is spiritual blindness. In addition, God will send upon them "strong delusion" which has the effect of their believing a lie and thinking it is the truth. The only way to be delivered from that strong delusion is through repentance by applying the atoning sacrifice of Christ in their lives.
"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 2:10-12