The Nature of Humans
Humans are Mortal
There is a famous quote from Benjamin Franklin that says "Nothing is certain except death and taxes." We are all acutely aware of the fact that humans, unlike God, are mortal and will die one day. We don't need the Bible to tell us that we are mortal. However, what is interesting is what the Bible says about what happens to us when we die.
"For the living know that they will die, but the dead know nothing, and they have no more reward, for the memory of them is forgotten. Their love and their hate and their envy have already perished, and forever they have no more share in all that is done under the sun...Whatever your hand finds to do, do it with your might, for there is no work or thought or knowledge or wisdom in Sheol [the grave], to which you are going." Ecc 9:5-6,10 (ESV)
"Turn, O LORD, deliver my life; save me for the sake of your steadfast love. For in death there is no remembrance of you; in Sheol who will give you praise?" Psa 6:4-5 (ESV)
"Put not your trust in princes, in a son of man, in whom there is no salvation. When his breath departs, he returns to the earth; on that very day his plans perish." Psa 146:3-4 (ESV)
The Bible makes it very clear that to be dead is to be completely unconscious. In fact the Bible often refers to death as a sleep. Now this seems morbid and depressing but we do have a hope. Our hope is not that we will continue to be active while we are dead, but rather that we can be raised up from the dead by God. We have a hope in resurrection to eternal life.
Humans are Sinful
But why are humans mortal? Why do we die? The answer is simple; God is life. He created us and he sustains all life. If we turn away from him then we cannot continue to live forever. And everyone has turned away from God (except the Lord Jesus). Disobeying God's commands is called sinning. Sometimes sin is obvious, for example God says "Do not steal" and so it is clear that if we steal something we are sinning. However, it can also mean not doing something that we should be doing. That is also sin. The relationship between sin and death is made very clear by the Bible.
"...the soul who sins shall die." Eze 18:4 (ESV)
"For the wages of sin is death, but the free gift of God is eternal life in Christ Jesus our Lord." Rom 6:23 (ESV)
"...for all have sinned and fall short of the glory of God..." Rom 3:23 (ESV)
We Are Responsible for our Sins
One very important aspect of human mortality is that everyone is responsible for their own actions. Our sins are our own. There is no external force that causes us to sin. This mechanism of our own sins resulting in our mortality is clearly laid out in the Bible.
"The soul who sins shall die. The son shall not suffer for the iniquity of the father, nor the father suffer for the iniquity of the son. The righteousness of the righteous shall be upon himself, and the wickedness of the wicked shall be upon himself." Eze 18:20 (ESV)
"Let no one say when he is tempted, “I am being tempted by God,” for God cannot be tempted with evil, and he himself tempts no one. But each person is tempted when he is lured and enticed by his own desire. Then desire when it has conceived gives birth to sin, and sin when it is fully grown brings forth death." Jas 1:13-15 (ESV)
"And he [Jesus] said to them, 'Then are you also without understanding? Do you not see that whatever goes into a person from outside cannot defile him...What comes out of a person is what defiles him. For from within, out of the heart of man, come evil thoughts, sexual immorality, theft, murder, adultery, coveting, wickedness, deceit, sensuality, envy, slander, pride, foolishness. All these evil things come from within, and they defile a person.'" Mark 7:18-23 (ESV)