Just as damning other people to Hell is a sin, deciding that everyone ever will be saved is a sin. The fact is, we don't know how God will judge. We can hope that He will save all, but that depends on the heart of the person, and we cannot say for sure how the Judgement will go down. It's very dangerous to assume who will be saved and who will not, and it is very dangerous to assume how many will be saved and how many will not.
Its pretty easy to understand how Gods judgments will go down, His name is Jesus. He reverse the sin of Adam.
Romans 5:18-20 (Weymouths)
It follows then just as the result of a single transgression is a condemnation which to the whole race, so also is the result of a single degree of righteousness is a life giving acquittal which extends to the whole race. (19) for as thought the disobedience of one individual the mass of mankind were constituted as sinners, so also though the obedience of one, the mass of mankind will be constituted righteous. Now law was brought in later on so that the transgression might increase, but sin increased, grace is overflowed.
1 Corinthians 15:22-24
http://www.biblegateway.com/bg_versions/bgclick.php?what=2http://www.biblegateway.com/bg_versions/bgclick.php?what=222For as in Adam all die, even so in Christ shall all be made alive.
23But every man in his own order: Christ the firstfruits; afterward they that are Christ's at his coming.