The Heart is Unreliable
We have all heard the adage "nobody is perfect." This is normally said in a situation where we want to justify our imperfections or minimize the imperfections of others. We say it to minimize sin. God has a different approach. God also tells us that nobody is perfect. However, God intends to maximize the issue of sin in human beings by telling us this. He shines a floodlight on our imperfections, our sin, our despondent and darkened natures. When God flooded the earth, do you remember why He did it? It is written, "And GOD saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually." (Genesis 6:5) Do you remember what God said after the flood, after He had destroyed those evil people? "And the LORD smelled a sweet savour; and the LORD said in His heart, I will not again curse the ground any more for man's sake; for the imagination of man's heart is evil from his youth; neither will I again smite any more every thing living, as I have done." (Genesis 8:21) People didn't change after the flood! You see, people are evil from the inside out. It is not the evil things that we do that cause us to be declared as evil. It is the wickedness and deceitful heart we have that leads us to doing evil things. Jesus also confirms that our very natures are evil when He says "But those things which proceed out of the mouth come forth from the heart; and they defile the man. For out of the heart proceed evil thoughts, murders, adulteries, fornications, thefts, false witness, blasphemies: These are the things which defile a man: but to eat with unwashen hands defileth not a man." (Matthew 15:18-20) We are broken so badly that we are unable to be acceptable to God on our own merits, no matter what they are.
Our imperfect, deceitful hearts have also made it impossible for us to fully trust our feelings when attempting to discern truth. The feelings which spring from our hearts are tainted by our personal desires, thoughts and imaginations. Proverbs 28:26 says, "He that trusteth in his own heart is a fool; but whoso walketh wisely, he shall be delivered."
Our hearts are an unreliable guide for finding and knowing truth.