You make a very salient point when you contrast the doctrines of Judaism and Islam that we earn our entrance to heaven through good works with the doctrine of Christianity that eternal life is bestowed by the grace of God to those who know Jesus Christ as their Savior (and who therefore rest in the assurance that He has given His life for the forgiveness of their sins).
Jews and Muslims, believing that they must perform good works in order to earn their way into eternity, must daily ask themselves, 'Am I doing as He would want me to do?' It seems to me that following that belief carries with it a terrible sense of 'insecurity' (for lack of a better word) and unsure-ness about one's eternal future. After all, none of us is faultless, and each of us who believes that heaven is 'earned' must, if we are critical and analytical thinkers (let alone possessors of a conscience), occasionally, anxiously, ask ourselves, 'Am I measuring up? Am I behaving well enough?' Such self-examining questions are potentially good, in that they serve to keep us answerable to an internal monitor. But it is when that internal monitor goes awry (in those in 'leadership' positions in our religion) that we need to be vigilant regarding those who would use their power and position to debase our (and our fathers') faith. Such is the case with radical Islam.
In contrast, the Christian, believing that God's grace, and Christ's sacrifice, promise him eternal life, needs only genuinely strive to live a Christ-like life, knowing full well that he will fall short (because of the concept of original sin, and because he knows himself to be not without fault) .... but that his God is forgiving. The Christian believes that man is only as 'good' as he is able to be successful in battling his own innate sinful nature.
Those who preach radical Islam use this earning one's way into heaven belief to their depraved benefit -- as is evidenced by the misguided (forgive the understatement) hijackers who apparently believed that their abominable actions earned them instant access to paradise. That anyone claiming to follow a 'religion' believes God would condone (let alone desire, and reward) the torturous deaths of thousands of innocents exhibits the inherent danger in earning God's favor -- the concept of 'earning' is open to human interpretation. And when the interpreters are madmen, who hold significant sway over millions, religion finds itself perverted into 'God'-inspired terrorism. How can the true follower of any God-based religion embrace the idea that his God would be pleased with the death and destruction of fellow humans, done in the name of Him who created us all?
I am in no way saying that Judaism, or foundational Islam, are not of God -- or that Jews or true Muslims are following false doctrine. Nor do I degrade or disrespect either religion's belief that one earns one's way into heaven. I simply believe that that belief must be carefully guarded, lest those madmen who do not seek to know God (and, worse yet, who seek to know Him only insofar as falsely claiming to perform His will will garner them more despotic power over others here on earth) be allowed to transform a God-based religion into a man-based abomination.