"Why do people try to gain salvation through their own merit, rather than accept God's righteousness through Jesus Christ? How strong a role does guilt play in this?"
This was the question posed to me. Here is my response.
From the beginning, angels and humans have inclined toward rejection of God as their ultimate Authority. God told Adam and Eve that they had for their food every plant in all the earth and every tree of the garden, except one. The serpent deceived Eve, pretending, without actually lying, to believe that God had forbade them any tree in the garden. The serpent's argument was that God's authority over them was arbitrary and unfair. The serpent opened a door into rebellion, making it appear good and delightful and wise, and Eve walked through that door. (Genesis 3:1-6)
Adam was beside Eve all this time. He knew that the serpent was pretending. Adam knew that the serpent was usurping authority over God, but he also knew that this was a pivotal moment that would establish whether humans were subject to a higher Power or were themselves Masters of their lives.
Adam knowingly rejected God's authority.
Accepting God's righteousness through Jesus Christ requires one to submit to Someone higher. Acknowledging my condition as sinful and depraved, and gratefully accepting God's offer of forgiveness through the sacrifice of His Son requires one to acknowledge God as Right and Supreme.
Why do humans tend to construct religions that offer eternal joy in exchange for righteous actions? Why do humans gladly accept the notion that their good works or outward behaviors gain them heaven? It allows them to feel in control of their destiny. It allows them to feel at least as "righteous" as God, and perhaps a bit more so. It allows humans to be Masters of their own lives.
A life spent balancing "sin" with "goodness" is at odds with one's God-given conscience. The person will oscillate between guilt and pride. Guilt is good, in that it provides a clear signal that something is wrong with us. However, it is pride that argues for self-justification and against God as Rescuer and Redeemer.
We don't try to gain salvation through our own merit because of guilt, but because of pride responding to that guilt. Pride says, "I know I've done wrong and God has the power to slay me, but I still have sufficient strength and wisdom to escape my doom. I can hide from God. I can atone for my sin through penance or good works. I'm not entirely at the mercy of God's judgement."
Adam and Eve's first response to guilt was to hide from God:
"They heard the sound of the Lord God walking in the garden in the cool of the day, and the man and his wife hid themselves from the presence of the Lord God among the trees of the garden. But the Lord God called to the man and said to him, Where are you? And he said, I heard the sound of you in the garden, and I was afraid, because I was naked, and I hid myself." Genesis 3:8-10 (ESV)
AFRAID: yare (to fear, revere, frighten)
NAKED: erom (nudity); from aram (to be or make bare, to be cunning in the sense of smoothness)
Adam was afraid of God because he rightly understood that he lacked even the smallest protection in the form of clothing. But Adam's nudity contained an element of rebellious cunning: "I may be weak compared to God, and I may be extremely vulnerable to injury or destruction at His hand, but at least I can hide...at least I can hold to a small portion of autonomy and self-esteem."
Adam felt guilt, rightly so, but his response was prideful. He accused Eve of forcing his disobedience, hoping to still escape God's justice without stooping to ask for mercy.
Adam lived for 930 years and the Bible offers no evidence that Adam ever did stoop to ask God for mercy. Eve apparently did submit to God's authority and appreciated His mercy:
"I have gotten a man with the help of the Lord...God has appointed for me another offspring instead of Abel, for Cain killed him." Genesis 4:1,25 (ESV)
Eve honored and thanked God for her children, seeing it as evidence of His mercy and providence. There is no evidence that her devotion bothered Adam...he did not forbid her worship or prevent her from expressing her worship of God. But there is no evidence that he joined with her in glorifying and enjoying God.
Adam was only human, after all.
http://www.flickr.com/photos/chant3/3228273137/sizes/m/in/photostream/">Image, courtesy of Dar'ya Sipyeykina