‘And for this cause God sends them a working of error that they should believe the lie, that they all might be judged who believed not the truth but had pleasure in unrighteousness.'
When men spurn God's Spirit the alternative is error. That was so in Genesis 6:2, and is always so. When Pharaoh hardened his heart, God also began to harden his heart, for on one who was hardened His activity could only produce hardness. The same is true here. They did not want truth, so when God continued to work in them turning their thoughts to spiritual things, it could only produce error. And instead of believing the truth, their perverted minds believed ‘the lie', the opposite of ‘the truth', teaching suffused with error presented by the man of sin.
This will be contrasted in 2 Thessalonians 2:13 with God's working of truth in the hearts of those who are His. Both are His sovereign work. He will make known the riches of His glory on vessels of mercy which He prepared beforehand unto glory, and endures with much longsuffering vessels of wrath fitted for destruction (Romans 9:22-23).
But what basically is ‘believing the lie'. It is to have the wrong attitude towards, and understanding of, God (Romans 1:25). To ignore the clear message of creation. It is to see that which is anti-God as being God, and to see God in what is merely part of this world. It is to listen to the whispering of Satan, as man first did in the Garden of Eden. And it leads to the worship of Nature and bestial lives (Romans 1:18-32).
But the result can only be that they will finally come under judgment, the judgment that is to follow the appearance of the man of sin. And that judgment will reveal that it was because they had pleasure in unrighteousness (‘delighted in wrongdoing') that they did not believe the truth, and because they continued not to believe the truth that they continued to have pleasure in unrighteousness. The two go together. Not believing the truth will always result in having pleasure in unrighteousness, and having pleasure in unrighteousness will always result in not believing the truth. And it is because men have pleasure in unrighteousness that they will follow the man of sin until the inevitable day when they will be judged.
This principle is very important. When a man begins to lose his faith ask him in what way he wants to misbehave. Soon you will discover that his problem is not a rational one but a moral one.