When discussing how to determine the truth and error of spiritual matters, people often make statements like the following: "I just follow my heart; I feel in my heart that this is right; I go with what my heart tells me; I trust my heart." Such statements sound fine to most people, but the Bible plainly states that "he who trusts in his own heart is a fool." 
Why is it foolish to trust one's own heart? Won't our heart tell us what is right? On the contrary, Jeremiah (17:9) tells us: "The heart is deceitful above all things, and desperately wicked: who can know it?"
If the Bible only said that our heart is deceitful, that would be bad enough. But it says more: the heart is deceitful "above all things." If "all things" truly includes all things, then our heart is more deceitful than the devil himself, which is a frightening thought.
Likewise, if the Bible only said that our heart is wicked, that would be bad enough. But it says more: the heart is "desperately wicked." Our wicked heart is so desperate to continue ruling our thoughts and our life that it will go to any and all lengths to keep from being exposed for what it is, "full of madness and folly (Eccl. 9:3)." In its desperation to survive, our heart is willing to masquerade as anything, even as a blood-washed, born-again Believer, heart in love with the Savior. Thus Jeremiah ends this verse with the question, "Who can know it?"
This diagnosis of the heart should also put the fear of God into anyone who takes the Bible seriously. Another sobering passage is 2 Thessalonians 2:7-12, where we read about what happens to those who refuse the truth: "God sends them a powerful delusion so that they will believe the lie." If we are deluded by the devil, or even by our own heart, we might hope for pity from God. But if God Himself is the one who sends the delusion, as a judgment for our refusal to love truth, what hope is there for deliverance from deception? Who can deliver us from a strong delusion that God has sent?
The problem with deception is that the deceived person has absolutely no idea that he is deceived. If he knows that he is deceived, then he is not deceived! Deception, by its very nature, is impossible to detect in one's self. Until the deceived person begins to see that he is deceived, he does not suspect that he is under strong delusion.
In Matthew 7:21-23, Yeshua describes a multitude of people who will not see their deception until it is too late for them:
"Not everyone who says to Me, 'Lord, Lord,' will enter the kingdom of heaven; but he who does the will of My Father who is in heaven. Many will say to Me in that day, 'Lord, Lord, did we not prophecy in Your name, and in Your name cast out demons, and in Your name perform many miracles?' And then I will declare to them, 'I never knew you; depart from Me, you who practice Lawlessness."
Two things about this passage should motivate Bible-believing Christians to examine their faith. The first thing is the fact that the people described here are not atheists or Hindus or Moslems. These are church people, involved in church activities, doing what appear to be good works in the name of Jesus. They even call Him "Lord," and fully expect to inherit eternal life through their relationship with Him. This is why they are stunned to find themselves excluded from the Kingdom. The second sobering thing in this message is the word "many." This is not some small group of cult leaders and false prophets; it is a multitude that the Lord describes as "many."
This should raise some frightening questions for a Believer: How do I know that I am not among this multitude of deceived people? Am I truly blood-bought and born from above, or is it only my own deceitful heart, in an act of desperation, pretending to be in love with the Lord? How do I know that my salvation experience was not just an imaginary experience, a strong delusion sent by God as judgment for my refusal to love the truth?
We know from the Scriptures that our heart cannot be trusted to tell us whether or not we are deceived. If we are deceived, and look to our heart to tell us, our heart will simply continue to lie to us and give us false assurance of our salvation and our doctrines, complete with warm, emotional, religious feelings to soothe any doubts we may have. And the longer we remain rooted in our deception, the stronger these feelings will be.
If we cannot trust our own heart to tell us whether or not we are deceived, who can we trust? A pastor or a prophet? A pastor or prophet might give us the words of assurance and comfort, but how do we know the pastor or prophet is not deceived? The Bible is full of warnings about false pastors and prophets. God told Ezekiel (14:9) that “if the prophet be deceived when he hath spoken a thing, I the LORD have deceived that prophet.” So even a prophet may be among those to whom the Lord sends powerful delusion!
Fortunately, we need not despair. The same Bible that warns us of deception and false teachings also tells us how to discern the true from the false. In Isaiah's day deception was rampant in Israel, and the Lord gave a simple formula for discerning truth from error: “To the Law and the testimony: if they speak not according to this word, it is because there is no light in them."  All true beliefs and teachings will be in agreement with Torah (the Law God gave to Moses); any belief or teaching that opposes or contradicts these commandments is not of the Light, but of the darkness.
If we look at the context of the passages discussed earlier in this paper, we will see that deception is, in these cases also, related to our attitude to God's Law. When the Messiah tells the multitude of deceived church people to depart, he addresses them as “you who practice Lawlessness.” Likewise, Paul's warning of the God-sent delusion in 2 Thessalonians is also presented in the context of our attitude to God's Law. The passage speaks of “the secret power of lawlessness ('separating from Torah,' Jewish New Testament),” which was "already at work" in Paul's time. Two times in this passage, Paul mentions "the lawless one ('man who avoids Torah,' Jewish NT)." Even Jeremiah's statements about the heart are soon followed by a lengthy plea to honor God by keeping the Sabbath.
Of course most Christians consider the Sabbath and many other commands to be, for the most part, irrelevant to those living under the New Covenant. Ironically, the Bible tells us that one important mark of true New Covenant believers is the keeping of God's Torah! In Jeremiah's well-known prophecy of the New Covenant (31:31-34), the Lord describes his intended outcome in the lives of all those who accept the terms of the New Covenant: "I will put My Law (Torah) in their inward parts, and write it on their hearts."
This new heart with God's Torah written upon it was also prophesied in Ezekiel 11:19f. Here the Lord describes the results of receiving a new heart: “Then they will follow my decrees and be careful to obey my laws." Ezekiel later writes about the promise of the Holy Spirit: "I will put My Spirit in you and MOVE YOU to follow My decrees and be careful to keep my laws.”  Torah observance is not learned and developed overnight, but if a believer is not being MOVED in a Torah-obedient direction, he should find out why not. According to Ezekiel, being moved to obey God's Torah is evidence of having received the Holy Spirit. Speaking in tongues (cited by Pentecostals as the evidence of the Spirit) is great, but tongues can be counterfeited. Obedience to Torah cannot. Either a person does it or he doesn't.
There are also many NT verses that point to Torah observance as a mark of true New Covenant believers:
"We know that we have come to know Him if we obey his commands. The man who says, 'I know him,' but does not do what He commands is a liar, and the truth is not in him."  "This is love for God: to obey His commands.”  "Keeping God's commandments is what counts."  In Rev. 12:17, God's faithful remnant is described as those "who obey God's commandments and hold to the testimony of Yeshua." In Rev. 14:12, "saints" are defined as "those who obey God's commandments and remain faithful to Yeshua."
A love for Torah is not proof that a person is not deceived in other areas, of course. But in these passages about deception, it is clear that deception is closely linked to our attitude to the Law. We must not turn a deaf ear to the Torah. Proverbs 28:9 says, "He that turneth away his ear from hearing the Law, even his prayer shall be abomination."
Any believer who feels any hostility toward God's Law should seriously consider the possibility that he may be under strong delusion, and ask God to forgive his lack of love for the truth. He should ask God to release him from the secret power of lawlessness. If God has sent strong delusion to someone for refusing to love the truth, God can "undeceive" the repentant heart that agrees to embrace the Torah that he formerly ignored or despised. "A broken and contrite heart, O God, Thou wilt not despise."