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?