“For my people have committed two evils; they have forsaken me the fountain of living waters, and hewed them out cisterns, broken cisterns, that can hold no water.” (Jer 2:13 ESV)
– Photo by Susanne Schuberth
Below you can read a definition of what the word ‘delusion’ basically signifies. T. Austin Sparks explained it this way.
What is delusion? Well, it is to lead from truth into error. It is to mislead the mind or the judgment, to beguile, to lead astray, to cheat, to frustrate, to disappoint, to impose upon; something that is not the truth; and these are days peculiarly marked by that peril. 
Actually, delusion and deception are so natural to us that we usually submit to them without knowing about it until God sets us free. On the one hand, there are people who are atheists or agnostics who are deceived because they reject The Truth which is Christ. On the other hand, there are religious people, even Christians, who are deceived because they are content with what they know and can do for God or other religious entities. These believers do not want to grow spiritually because it is too painful for the flesh which must die and make room for Christ in us. Furthermore, there are Christians who have come to know parts of the truth and who had their personal experiences with God and Christ too, however, they habitually eat the whole meat of everything they were taught and do not spit out the bones. Thus it is often times a horrible mix of truth and lies they offer and you need to be cautious to not get captivated by their confusing spiritual spider webs, either.