Total Depravity, the first of the 5 points of Calvinism.
This refers to original sin, the first sin of rebellion carried out in the garden of Eden that corrupted man’s nature. How did this come about?
For starters, humans were not the first beings created. Before creating humans, God created all other life including more spirit-type beings like the various casts of angels. Chief among them was Lucifer. Lucifer was the first to turn selfish and thereby wanted himself to be at the center of things, so he rebelled against God and there were other angels that followed him. He then proceeded to aim to bring the very pinnacle of God’s creation into his rebellion (the first man and woman) and therefore proceeded to tempt (in the form of a serpent) Eve and thereby tempt Adam to rebel against God’s one rule, to not eat of the tree of the knowledge of good and evil.
Before this, mankind was in a state of innocence. God, who has not changed from then even until now, is holy and just by nature and could not go back on the punishment he promised; therefore, the curse of sin (which brings death) fell upon the first two humans. There was no more innocence, they now had every thought, desire, and action tainted by sinful desire that focuses on self rather than God. In such a state, mankind could no longer be the image bearers they are meant to be. Adam and Eve became like marred and cracked mirrors and through them the rest of creation became subject to the curse of sin that all began with rebellion to God.
So where does that get us to now? Well, from Adam and Eve was born the human race. In their corrupted state, they could only ever give birth to corrupted beings like themselves. As such, we are born into a depraved state that has no natural inclination towards anything in particular besides self. We are therefore totally depraved of Him.
This does not mean we are unable to do any “good” actions without God in our lives but it does mean each of those supposedly “good” actions without Him will actually be all about ourselves and how good we are. In other words, the “good” actions carried out by the unbelieving world are ultimately bad because they do not seek to bring God glory but instead seek to bring self glory (much as Lucifer sought to do with himself who loves to masquerade as an angel of light).
This then sets us up for the coming points of the TULIP tool, next of which is Unconditional election.
If you want to preempt me, here is a link to a list of articles and the like on the 5 points of Calvinism.