Why can't there be another fall? Some disagree on whether a Christian in this life may fall utterly and lose or forfeit eternal life.  However, most Christians do agree that after death, the Christian is eternally secure in heaven (or the new earth).  And this seems to be well supported in Scripture:

John 10:28 (ESV)
28   I give them eternal life, and  they will never perish, and  no one will snatch them out of my hand.
Revelation 21:4 (ESV)
4   He will wipe away every tear from their eyes, and  death shall be no more,  neither shall there be mourning, nor crying, nor pain anymore, for the former things have passed away.”

So it seems clear that Christians could not be susceptible to another Fall.  My question is, Why?  What is the fundamental reason why Christians would not be susceptible to another Fall or rebellion against God?
Please answer from a Protestant, non-Calvinist perspective.

Possible responses I have considered

There will be no law, and thus no concept of sin

But wouldn't rebellion against God would be considered sin, even apart from a &quot;law&quot;?

After death we no longer have a sin nature, and are thus incapable of sin

But weren't Adam and Eve created without a sin nature, and yet sinned?

Satan will be vanquished, unable to tempt us

But didn't Lucifer rebel without being externally tempted? If we are unable to rebel without a tempter, that implies that we will have less free will than Lucifer and the angels had.

We will be unable to sin, either through lack of free will, or prevention by God

The argument that I usually use and hear for the existence of free will is that God would rather have willful obedience than robotic obedience.  Is God then hedging on this preference for the sake of our eternal souls?

We have already been atoned for by Christ, so if we were to sin, it could not be counted against us

This allows for sin in heaven, which I can't buy.  It contradicts Revelation 21:4 for one thing, and makes heaven imperfect
"If any man be in Christ, he is a new creation."  Saved human beings have an old nature and a new nature.  At death, the old nature is removed entirely, and only the new nature remains.  (This is the "glorification" in the salvation trio of justification, sanctification, and glorification.)  You should not think that this new nature is a return to the nature that Adam and Eve had before the Fall.  Whatever that nature was like, it is gone forever.  "Behold, I make all things new."  

One other point: angels have another nature altogether, so making comparisons between humans and angels is problematic.  For example, we are told (Hebrews?) that angels don't understand salvation.  They long to look into salvation (so they can understand and worship God better, I suppose), but it escapes them.  We don't know why.  Some hypothesize that they are incapable of repentance, which explains why (apparently) God never redeemed any fallen angels and why (apparently) angels don't fall from time to time.  Once angels choose (goes the theory), they have chosen, and they will never choose otherwise.  It is not a lack of free will; changing their minds is simply not in their nature.