Once people reject the truth or decide for themselves what the truth is, what is there? What is left? What does it produce? Fables, myths, manmade wisdom, unsound teaching. It is certainly not the pure truth from God, and so Paul writes, "They turn their ears away from the truth and are turned aside to fables."
"Fables" is a general catchword for anything that is not the truth. When we are not focused on God and those He has sent to preach the truth, we are dabbling in error. What happens when one pours poison into a drink? Is it a good drink any longer? It is poison! Any amount of poison in that drink means that it is no longer pure. And any truth that has error mixed within it is an error. That may seem to some like a hard saying, but God wants His truth preached purely. So, we have to be careful, do our best, to feed ourselves the unadulterated truth. Otherwise, we will find ourselves turned away to fables, to myths, to manmade wisdom, to false gospels.
In Romans 1:18-25, the apostle gives a quick summary of what happens when truth and error are mixed. He asserts that the truth is out there. People are without excuse because God's message is readily available. The truth is knowable, but men have suppressed it—in certain places, in part, and in other places, almost fully. What they have mainly done is to add their own "wisdom" to the mix.
They have thus made their own religions. Why? They satisfy them in some way. God just replies, "Okay. If that's the way you want it, go ahead" (Romans 1:24, 26, 28). It is like Jesus' attitude to those who rejected the truth that His disciples preached: "Shake the dust off your skirt and go somewhere else" (Matthew 10:14). He essentially instructs them, "Don't worry about the naysayers. Leave them be. I'll deal with them later. Move on."
The greater point we need to understand is how they did it. These unrighteous people suppressed the truth, adding their own bit of "wisdom" or supposed knowledge or truth—and they immediately became fools because they believed a lie. They thought they could come up with a religion better than the true religion from God. And they will answer for it in time.
But we have to be careful the same thing does not happen among ourselves:
The coming of the lawless one is according to the working of Satan, with all power, signs, and lying wonders, 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. (I Thessalonians 2:9-12)
This prophecy returns to what Paul said is the foundational reason for false gospels and false teachings. It is what he writes in the last phrase: They took "pleasure in unrighteousness." It lined up with their own lusts. They wanted their itches scratched, and so they believed the lie—Satan's lie.