It means that from God's perspective, these people had the truth presented to them and they didn't love it. It does not mean that these people did not agree with it, it means that they didn't love it. What does it mean (in the light of that) when God gives them strong delusion? There are answers to this right in the scriptures; we'll just turn to one.
When it says that God sends a delusion, it means that He quits (at this point) trying to save them, and gives them over to their own desires. So they placed their delight—their desires—in unrighteousness. What is Paul admonishing these people [to do], in a practical way—in some way that you and I can profit from? What he is saying brethren, is in the kind of situation in which we are dealing, a Christian cannot be neutral.
In Romans 1:22-25, Paul describes the unrighteous as those who change the glory of the incorruptible God into an image made like corruptible man. Paul calls them fools. He says they exchange the truth of God for the lie. Trying to make God like a man is a lie. And foolish.
There is a lie that is above all other lies. It is the belief that something or someone is to be venerated and therefore obeyed in the place of the Person and truth of God. That is the essence of idolatry. Most of the time, the one that we do that to and for is our self.
