deceitfulness of the heart, deception, delusion, discerning the spirits, entering his rest, exhortation, experience, false prophets, God's love, Jesus Christ, Satan, sin, spiritual growth, T.A. Sparks
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 as well.
Just lately I discovered a new trend which cannot be observed publicly on the net because it happened on a more private basis. As a blogger I detected this strange trend either through comments I did not moderate, through emails, or through phone calls. Indeed, there are some Christians who feel urged by ‘God’s voice’ to tell you ‘the truth’. In particular, these ‘prophetic voices’ claim to have heard God and feel forced to tell you what is wrong with you and what you should do better in ‘God’s sight’. If I had no close relationship with God myself, I could be easily influenced by their self-centered and confused ‘messages’. Brothers and sisters, it is always good to search our own hearts and to ask God what He thinks about our doing or not doing and I do that, daily. And I believe that God did not send prophet(esse)s and watch(wo)men who speak up for God unless they have experienced the cross of Christ (i.e. dying to self) in their own lives. Not having gained the real knowledge (i.e., love) of God and genuine self-knowledge, they do not even realize that it is their own deceived heart and mind that causes them to confuse God’s and Satan’s voice. They feel some ‘power’ to open up their mouth and think it must be God’s power. Yet God only shares His resurrection power with those who are weak in their flesh, not with those who are still strong regarding their carnality and self-willed schemes. Resurrection comes after death, right? Not the other way round. Often times, these people refuse to be part of the Body of Christ too because they want to have it all their way. This is not the way it should be. The Body has many different members and we all should submit to one another according to what God has given each of us. But if He did not give somebody anything, we need to resist the devil’s voice who wants to make us submit to himself again. In fact, if we search for someone who has not been, at least partly, deceived, we should not be disappointed if we hardly find anyone. Sparks in this same article  came straight to the point as he said,
The race is deceived universally. Deception is not something now that is out in the air, objective, apart from man; deception is now something in man as a part of him. “The heart is deceitful above all things, and desperately wicked.”
Beloved, deceitfulness was not a constituent in the man that God made. Deceitfulness is something that has come into the nature of man, a foreign something, and it is in the race. It is in man, it is a part of his very nature. God’s greatest opportunity is always with the honest in heart, and God never stands a chance when there is not honesty of heart. His beginnings are there. A contrite heart, an honest heart, the open spirit, where there is no covering things over; for that was the very first thing that happened when deceitfulness found a lodgment in the race, man went to hide himself, and deceitfulness is always that which is hiding something, keeping something under cover, not transparent, clear, open before heaven. God always demands absolute openness and honesty before He begins negotiations, and God can always get on very rapidly when He has an honest heart to deal with, and that life with an honest heart will go ahead with God by leaps and bounds, and those who will not be honest with God, meaning business whatever it costs, will make no progress.
Sparks’ solution for quick spiritual growth and freedom from deception is quite simple, isn’t it? Honesty. Indeed, this is the best policy, both with God and with our fellow man. If we no longer try to be better than we really are but present us just as we are right now, God can and will help us. Don’t we think that it is shameful to find out that we have been betrayed, or even, that we in our deception deceived others too (maybe only unknowingly). It is never too late to return to the Lord as long as it is called TODAY. In fact, hiding sin before God and/or others opens up our hearts for deception. Instead, we need to walk in the light as He is in the light, with God and with one another. Sin in itself has a deceiving effect because its author Satan is The Deceiver who won’t omit an opportunity to tempt us in any way possible. I was thinking here of an admonition taken from the letter to the Hebrews that expresses it perfectly.
Therefore, as the Holy Spirit says, “Today, if you hear his voice, do not harden your hearts as in the rebellion, on the day of testing in the wilderness, where your fathers put me to the test and saw my works for forty years. Therefore I was provoked with that generation, and said, ‘They always go astray in their heart; they have not known my ways.’ As I swore in my wrath, ‘They shall not enter my rest.’” Take care, brothers, lest there be in any of you an evil, unbelieving heart, leading you to fall away from the living God. But exhort one another every day, as long as it is called “today,” that none of you may be hardened by the deceitfulness of sin. (Heb 3:7-13 ESV)
T. Austin Sparks also said, “Christ is God’s Sabbath. It is in Christ that God comes to rest in all His works; His new creation.”  It is the same for us, dear brothers and sisters. We finally come to rest from our own works when we are found IN Christ, IN God’s new creation “for whoever has entered God’s rest has also rested from his works as God did from his.” (Heb 4:10 ESV) Deception and delusion often have to do with hurrying, with trying to quickly find out ‘the truth’, with the desire to appear more mature or more spiritual than we really are, or with wanting to be someone who acts on God’s behalf. Scripture confirms that God who is never in a hurry does not want to see us running to and fro. Instead, the Prophet Isaiah wrote,
“For thus said the Lord God, the Holy One of Israel, “In returning and rest you shall be saved; in quietness and in trust shall be your strength.” But you were unwilling, and you said, “No! We will flee upon horses”; therefore you shall flee away; and, “We will ride upon swift steeds”; therefore your pursuers shall be swift.” (Is 30:15-16 ESV)
May God give us the grace to enter His rest. Amen.