Biblical Evidence that the Totality of Man's Internal Thought-world is Depraved:
Man is made up of body and soul. The soul is identical to the heart. Like the different side and angles of a diamond, the soul, the spirit, the heart, are all different angles to view the same thing: the spiritual, internal thought-world of man. "The spirit" appears to be the internal thought-world of man in reference to God. "The soul" appears to be the same thing but in reference to the created order. And "the heart" appears to be its reference to itself. And yet, the Bible very clearly tells us every aspect of the internal thought world of man is fallen. Natural man's appetites, judgments, and will; his dreams, his drive, his reason, his imagination, his hopes, his actions, are all warped by our rebellion. There is no part of man that isn't rebellious. And there is no part of man that is acceptable to and before the sinless God of Israel.
The heart of man: Mark 7:20-23 And he said, "What comes out of a person is what defiles him. (21) For from within, out of the heart of man, come evil thoughts, sexual immorality, theft, murder, adultery, (22) coveting, wickedness, deceit, sensuality, envy, slander, pride, foolishness. (23) All these evil things come from within, and they defile a person."
The appetite of man: Proverbs 27:20 Sheol and Abaddon are never satisfied, and never satisfied are the eyes of man.
The judgment of man: Romans 1:28 And since they did not see fit to acknowledge God, God gave them up to a debased mind to do what ought not to be done.
The will of man: Ecclesiastes 2:10-11 And whatever my eyes desired I did not keep from them. I kept my heart from no pleasure, for my heart found pleasure in all my toil, and this was my reward for all my toil. (11) Then I considered all that my hands had done and the toil I had expended in doing it, and behold, all was vanity and a striving after wind, and there was nothing to be gained under the sun.
The reason of man: 1 Corinthians 2:14-15 The natural person does not accept the things of the Spirit of God, for they are folly to him, and he is not able to understand them because they are spiritually discerned. (15) The spiritual person judges all things, but is himself to be judged by no one.
His imagination: Genesis 6:5 The LORD saw that the wickedness of man was great in the earth, and that every intention of the thoughts of his heart was only evil continually.
His hopes: Ecclesiastes 7:29 See, this alone I found, that God made man upright, but they have sought out many schemes.
His actions: Mark 7:20-23 And he said, "What comes out of a person is what defiles him. (21) For from within, out of the heart of man, come evil thoughts, sexual immorality, theft, murder, adultery, (22) coveting, wickedness, deceit, sensuality, envy, slander, pride, foolishness. (23) All these evil things come from within, and they defile a person."
Is there anything in man that is upright? No. You might be thinking, "since I can't trust myself, therefore I ought to trust others." Except they are sinners too. And the Bible says in Romans 3:4 "Let God be true though every one were a liar …" The doctrine of Total Depravity leads to one unassailable conclusion: Trust not man (including yourself - you cannot save yourself; you are not good, and you are not God); but trust the Lord (the Father, Son and Holy Spirit), and his Word.
Total Depravity, the Gospel, and the Trinity:
Is this doctrine non-essential to the Christian faith? Are we just making mountains out of mole hills, fighting over small points of doctrine? The Gospel, in Greek, is "euangelion," that is "the good news." In contrast, Totally, or Comprehensive, Depravity is the bad news. In fact, you cannot have good news without first having bad news. You cannot be saved unless you first ask "saved from what?" Therefore, the doctrine of Total Depravity is necessary to the gospel. It protects our absolute need for it. It guards our every-moment-neediness of Christ. Further, God himself wants you to relate to him in light of this spiritual reality. In your flesh, you are not good. God is good. The Father has judged you to be a comprehensive sinner (Romans 3:23). The Son has come to rescue you from your sin (John 3:16). And the Holy Spirit was sent to convince you of your sinfulness (the bad news) and lead you to know your neediness of Christ's work for sinners (John 16:8-11). There is only one question left: do you reject any goodness in you, and your neediness of a Christ to save you from your sins?