Jeremiah 17:9 warns that the human heart is so desperately wicked that who can know it. From the King James Study Bible says this about the human heart:
"The heart (cf. Proverbs 4:23) means man's most innermost being. The bent of man's natural disposition apart from God's redeeming grace is described as deceitful (lit., crooked) and desperately wicked (incurably sick). Therefore man cannot trust his own heart but must leave all to God who alone knows the heart and judges all men fairly. Only a person with a redeemed heart can live with proper fellowship with God (Job 11:13, 1 John 3:18-24)."
Matthew Henry also stated this about the human heart on his commentary on Jeremiah 7:5-11 which I am presenting this excerpt:
"The heart, the conscience of man, in his corrupt and fallen state, is deceitful above all things. It calls evil good and good evil and cries peace to those whom it does not belong. Herein the heart is desperately wicked; it is deadly, it s desperate. Herein is the heart desperately wicked; it is deadly, it id desperate. The case is bad indeed, if the conscience which should be set right the errors of other faculties is a leader in the delusion. We cannot know our own hearts nor what they will do in an hour of temptation. Who can understand his errors? Much less can we know the hearts of others or depend upon them."
Hebrews 4:12 calls the Word of God as something more powerful than a two edged sword as it pierces, divides the soul and spirit, even revealing the innermost thoughts. The King James Study Bible says about the Word of God:
"For the Word of God: The reason given for one's careful scrutiny of his life involves the reality that God is intently scrutinizing it. His word is quick (living). This word is first in the Greek and is the emphasis. God's Word is not old or archaic; it is alive. It is not inept or inactive; it is powerful (active). It reaches to the inner secrets of man's mind to discern even his thoughts and intents. Likewise, God's eye sees man as though he were naked, unable to hide behind excuse or pretense."
Now I'll take something again from Matthew Henry's Concise Commentary on Hebrews 4:11-16:
"The Holy Scriptures are the Word of God. When God sets it home by His Spirit, it convinces powerfully, converts powerfully, and comforts powerfully. It makes a soul that has long been proud to be humble and a perverse spirit to be meek and obedient. Sinful habits that are become as it were natural to the soul and rooted deeply in it are separated cut off by this sword. It will discover to men their thoughts and purposes, the vileness of many, the bad principles they are moved by, the sinful ends they act to. The word will show the sinner all that in his heart."
We can see from the Parable of the Sower the four different kinds of soil which represents four different kinds of heart. There is the stony ground hearer, the shallow ground hearer, the thorny ground hearer and the good soil hearer. Trying to understand the Parable of the Sower shows what's wrong with today's evangelism methods and that before the Gospel is presented, the person must first be shown that they are sinners or their hearts must first be broken.
It's natural for any man who has been broken to be afraid of his own natural heart. In the Gospel According to Jesus, Revised and Expanded Edition, p. 135 we read this statement by Dr. John F. MacArthur:
"Nevertheless be encouraged. The Lord of the harvest can break up even the hardest ground and rid it of the most stubborn weeds (thorns). Hard soil, shallow soil, or weedy (thorny, emphasis mine) soil may not always stay that way. God can till the soil of the most stubborn heart. One ancient Palestinian method of farming was to throw the seed first then plow it under. That sometimes happens in evangelism. We sow the seed, and even when it seems that that the hovering birds are about to snatch it away, the Holy Spirit plows it under so it can spout and bear glorious fruit."
The whole reality that the human heart is so wicked that Spurgeon in the quote calls it a dark cellar full of loathsome creatures. That is what is revealed when the Law looks at our heart. Going back to the parable of the sower, the heart is so full of stuff that makes a man so stubborn and nobody really starts out as a good soil hearer. Many start off as worldly, hardened in sin or they are just shallow hearers. Some people are false converts before they become true converts. Some had to lose their fortunes before they would respond to the Gospel. Some rich men didn't want to get saved until they realized that their money can't buy salvation. Some people used to love sin a lot but later saw that they were not saved and they became good soil hearers. That is the reason why the Law must be presented because it is the Law that shows man the problem of their hearts and their need for salvation. Remember, some people have been witnessed to more than once before they got saved. In my own experience, I was witnessed to several times until I saw myself as a sinner only deserving Hell.
Putting aside the Protestant Reformers or the Protestant Reformation for awhile, remember that their methods of using the Law to witness was not their own invention but it's Biblical evangelism. When Martin Luther called the Law a hammer, he was quoting Jeremiah 23:29 that described God's Word like a hammer and it was always like a hammer even before he stated that truth. When Jesus witnessed to people, He always showed them their sinfulness before telling them the good news (John 4). When the Law is presented, people see themselves, Deuteronomy 31:26 and Romans 3:19 presents the sad, awful truth that we are all guilty before God. That was what was upheld by the pre-Reformation Baptists and that was even upheld by the Reformers. During the Protestant Reformation from Luther to John Calvin, they used God's Law to expose to people their sins before they told them the good news. Besides, some of the staunchest Calvinists would say, "Calvinism came before Calvin." although my non-Calvinist brothers and sisters in Christ may disagree but we can agree on this statement by Calvin saying, "Any teeaching that urges man to trust in himself merely a deception."
It's stupid but true that a lot of Protestant and Baptist churches today are just Protestant or Baptist in name but not in doctrine and practice. In short, most of them are just nominal Christians and not real Christians. If churches teaching conditional security gets me upset then so do churches that teach Antinomianism. The Antinomian is no more no less a false convert than the person who believes good works must be maintained to stay saved rather than good works are maintained because the person is saved. Worse, they are quick to say that those who say that true faith results to works is a contradiction to the doctrine of faith alone. Many churches today even do easy evangelism telling people they are sinners and making them say uh-huh rather than using God's Law to reveal that they are sinners. It would not make sense to the person they are sinners unless they first understand that they are sinners.