the answer you give is not convincing i feel.
for i think you tend to say that how do i know that mother teresa was really a good personality.!!
leaving apart lets think
" a noble person is there who is really pure inside out has never commited any sin and works for humanity but declines to adopt islam for he thinks either his faith is sufficient or is not convinced by quran than he would go to hell or heaven."
As far as my definition of righteousness is concerned, it is totally in line with the Quran,which i posted above, but even then it does not mean that i have the authority to go on assigning hell or heaven to people, simple as that.
The verses are all i can give to show you what the concept is, which is self evident and shows that believing in One God and the day of judgment, the Angels, the Divine Books, and the Prophets is
Mandatory for the virtue to be weighed as a virtue. BUT,
Suppose, you were a non believer till this time, and you've done many virtues till now, you might think that your previous virtues may not count if you were to accept the message of Islam now, but no, if you accept the message of Islam, all your previous virtues will still get counted. In addition, all your previous sins will be washed away, you start anew, from scratch.This has been said in the verses too, but I'll have to look for them to quote if you want me to.
As for maintaining the spirit of goodness without the belief in God, I'd ask whats stopping you / peer pressure, society, fear of abandonment etc.( or satan, how would you know its not satan)? Wouldn't that mean that You fear all those things more than you fear the one true God?One has to know God more, and the best way is through his own words, you will have to listen to more lectures on Islam to understand more.(you as in, whoever wants clarification on that concept).
The awakening or the need to know God usually happens when people have achieved a certain maturity in life and start pondering over the deeper meanings of life, once you've had a nice house, a car and a kid, you'd know what a purposeful life actually means. You could search for a God when younger too, but your sentimental attachment to your previous set of beliefs , or your loved ones can significantly contribute to steer you away from the truth.
Not believing in one God will keep you away from the final truth of life, and preferring to live in a comfortable lie isnt a 'good' justified', or a 'well suited for long' code of belief, it never lasts long. There are virtues in Islam that you cannot follow if you did not believe in God as He is.
Sura Baqara' Whenever my obedient ones ask you about me(Allah),say, I am near, I answer the appeal of the prayerful one, whenever he appeals to me, Let them respond to me and believe in me, so they maybe directed."(186)