"The coming of the lawless one will be accompanied by the working of Satan, with every kind of power, sign, and false wonder, and with every wicked deception directed against those who are perishing, because they refused the love of the truth that would have saved them. For this reason God will send them a powerful delusion so that they believe the lie"
2 Thes 2:9-11
The "love of the truth" is the essential gospel. You could describe it as the overall shape of the gospel. It's not complicated. It's accessible to everyone - regardless of intellectual capacity.
Jesus describes that shape clearly in places like John chapters 14 - 17 and in His sermon on the mount in Matt 5 - 7.
Knowing and understanding that shape is important. Because it's being bent and transformed in different ways today. If you lose that, you open yourself up to delusion. So warnings like the one above are also directed at those who are "within the household of faith" (Matt 24:24).
