It’s an altruism—anything you do long enough, you’re going to become proficient at. The same is true of your attitude toward God. If you reject the Gospel long enough, you become a professional Gospel-rejecter. Let me illustrate.
Have you ever seen the charred stump of a pine tree? What do you think happened? Probably someone cut down the pine tree. Then a fire burned the area and caught the stump aflame. After a while it burned itself out. In a year or so the underbrush grew up again. Then there was another fire. And this time, that stump did not burn as bright or as long. One day, there came a time when no matter how hot the fire was, it simply jumped over the stump.
First Timothy 4:2 speaks of those whose conscience is seared with a hot iron. Their hearts are charred and hardened against the Gospel. “Today if ye will hear his voice, harden not your hearts” (Hebrews 3:15).