What is the nature of eternal security? Jon Ericson made a statement in this answer that is fascinating to me:

In other words, we aren't secure in our salvation because of the nature of salvation, but because of the nature of Jesus.

This is something that I've wondered about, but have never really formulated.  Among those who hold the doctrine of eternal security, what is considered the nature, or origin, of that security?
Some possible viewpoints that I see are:

Salvation is eternal in its very nature.  This implies that abiding in Christ is evidence of salvation
Salvation is not eternal in nature, but is eternal in practice because of God's keeping power.  In other words, though theoretically possible to forsake Christ and thus forsake salvation, God promises to keep us, never allowing us to do so.  This would seem to imply that abiding in Christ is the cause, or medium, of salvation, but the work behind that cause is performed by God.

What are the Biblical arguments for or against these viewpoints?
While there are probably an infinite number of aspects in regards to eternal security, I would like to discuss it from a stance of imputed righteousness.


  Or do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived: neither the sexually immoral, nor idolaters, nor adulterers, nor men who practice homosexuality, 10 nor thieves, nor the greedy, nor drunkards, nor revilers, nor swindlers will inherit the kingdom of God.
  
  1 Corinthians 6:9-10 (ESV)


If we look at this verse as a condemning statement we would all fall under one or more of these classifications that will not inherit the kingdom of God.  (our sinful nature implies that we all have idolatrous hearts)  I think that is the reason why this passage is often preached as a sermon on making changes in your life and doing works.  But, the pivotal point of these verses is that the unrighteous will not inherit the kingdom of God.

This leads me to the main point that I am trying to make.  When we accept Jesus Christ as our savior we inherit his righteousness and are consider justified in eyes of God.  It is a common misconception that God "forgets" our sins when we accept Jesus as our savior.  If that were true then God would not be omniscient.  Instead God chooses to not remember our sins.  In the same sense that God remembered his covenant with Abraham, Isaac, and Jacob.  Instead, our sins are paid for with Jesus's righteousness, and the only way to obtain righteousness is through Jesus.  


  I do not nullify the grace of God, for if righteousness were through the law, then Christ died for no purpose.
  
  Galatians 2:21


So, logically we can say that we are eternally secure because:
1. The righteous will inherit the kingdom of God.
2. We have imputed righteousness at the time of salvation.