"The fallen man and the serpent are today so ONE that you cannot distinguish between them. I will go into this in detail in future articles on THE SERPENT, but suffice it to say that it is most enlightening to read the words of Jeremiah who said under the inspiration of the Holy Spirit that "the heart is deceitful ABOVE ALL THINGS, and desperately wicked: who can know it?" (Jer. 17:9). We should not find this so amazing a passage of Scripture if it were not for one important thing. Jeremiah did not list an exception, saying, "The heart is deceitful above all things, PERIOD! Since Jeremiah spake by the Spirit of God this could not possibly have been a slip of the tongue or something uttered before it was thought through. If the heart is deceitful above all things, it naturally follows that there is nothing more deceitful. The heart of man, then, is the MOST DECEITFUL THING IN THE WORLD!"

"A friend has shared the following inspired observations: “There is no doubt whatever in my mind that Jesus had this very scripture in mind when He spoke the words recorded in Mk. 7:15-23. ‘There is NOTHING,’ He said, ‘from WITHOUT A MAN that entering into him can defile him: but the things which come out of him, these are they that defile the man.’ Having said that, He uttered the statement that so often followed His teachings when there was contained in them a MYSTERY. ‘IF any man have ears to hear, let him hear.’ If we will be honest with ourselves, we will have to admit that we have fostered and taught for generations a concept that directly contradicts the words of Jesus Christ, for we have, indeed, declared that there IS something from without a man that can enter into him and defile him. We have called him satan (an adversary), and so he is! We have called him a murdered and a liar, and so he is! We have had much truth about him – but the one thing we have NOT known about him is his LOCATION! We have said that he was without – Jesus said that he is within! If there is NOTHING from without a man (and in the Greek that reads: ‘not one thing’) that entering into him can defile him, then we must conclude that satan's activity is not primarily without but within."

"As we continue to read this passage, we hear Jesus say, ‘For FROM WITHIN, OUT OF THE HEART OF MEN, proceed evil thoughts, adulteries, fornications, murders, thefts, coveteousness, wickedness, deceit, lasciviousness, an evil eye, blasphemy, pride, foolishness: all these things come FROM WITHIN and defile the man.’ Every sin category imaginable is contained in this discourse. Everything that a man could ever think or do that is evil is said by Jesus to come - not from outside of man but from within! In our consideration of this passage, there is one all important phrase that we must be careful to note, for it is the key to the true nature of satan and the heart of man. Jesus said, ‘For from within out of the HEART OF MEN...’ It is imperative that we notice the grammatical number of the two words here. The word ‘heart’ is singular being in the Greek in the genitive SINGULAR case. The word ‘men’ however is plural being in the genitive PLURAL case. We have, then, ONE HEART – but many men! This heart is the heart of the corruptible soul life, the self, the inward nature of man inherited from fallen Adam, one great nature shared by all the inhabitants of the earth, the very heart of Adam shared with his many-membered body!”

Much more in the link:

https://www.godfire.net/eby/Echoes/echoes06.htm