Web Address - http://www.rationalchristianity.net/perfection.html

Why won't good works get us into heaven?
    or, Why does God condemn "good people" who simply don't believe in Jesus?
 
In order to get into heaven - which means living in the presence of a morally perfect God who abhors evil - one must be perfectly righteous. Most people understand this to mean not doing anything bad, but righteousness is more than not doing what is obviously wrong. A perfectly righteous person not only doesn't do wrong, they don't want to do wrong. Not only does a righteous person avoid hurting people in anger or having affairs, they avoid fantasizing about these things (Mt 5:21-22, 27-28).
 
Skeptics often respond that these are "thought crimes," i.e. that this standard condemns us for our emotional reactions and stray thoughts which are out of our control. This is not correct: what is at issue is what we choose to think about or fantasize about. Something may cause us to feel angry, but nothing forces us to fixate on that cause until we become bitter and hateful. The thought of doing something evil may occur to us, but nothing prevents us from choosing to ignore that thought and focus on better things. (See also How can God require us to love him?)
 
Others object that thoughts cannot be considered wrong, for they are merely thoughts - "harmless fantasies" - and not actions. Yet actions begin with thoughts; an action is simply a thought carried out. Even if one never intends to do the things one fantasizes about, thinking about them will make one more likely to do them if the opportunity arises. If a married person secretly fantasizes about a co-worker, they're much less likely to resist if the co-worker makes advances towards them. If a person is angry and imagines making cutting remarks, those remarks are more likely to slip into their speech when they next speak to the person they're angry with (and they're more likely to be in a negative, emotional state for having focused on whatever made them angry).
 
Perfect righteousness additionally includes doing as much good as one is capable of. In other words, if one could have done good by giving to the needy, visiting the sick and elderly, etc. but didn't, one has committed a sin of omission by failing to do good (cf. 1 Sam 12:23).
 
If one commits even a single wrong, one can never obtain perfect righteousness. Not only has one fallen short of the standard of perfection, one can never do enough good to make up for the wrong. The wrong action can't be undone; hurtful words can't be unsaid, nor can physically or emotionally painful experiences be removed from a person's memory. There are no extra acts of kindness or goodness that can be performed for extra credit, for the perfectly righteous person would have been doing those acts all along. The time spent committing the wrong action could have been spent doing good instead, but now that it's past that time can't be retrieved and used for something else.
 
This is the problem of sin. Everyone has done wrong, so none of us are able to stand before God on our own merits. But God loves us enough that he took it upon himself to solve this problem: in the person of Jesus Christ, he bore the punishment for all our sins, enabling him to offer us forgiveness. And God's forgiveness removes the sins of commission and omission that prevented us from being perfectly righteous, so that we may enter heaven and be in the presence of God.
 
Thus, even someone that we would call a "good person" can't enter heaven on their own merits, but only by accepting God's grace offered through Christ. Those who reject Christ are rejecting God's forgiveness for their sins, so their sin still prevents them from entering heaven and still merits punishment.
 
Other responses (offsite) 
•Jesus: the Antidote by Gregory Koukl explains why those who reject Christ can't enter heaven
•I've lived a pretty good life. I'll go to heaven, if there is one, won't I? from FaithFacts
•Saving Faith by Dick Tripp - "The kind of faith that will get you into heaven."
 
Related articles 
•How can God require us to love him?
•Why did Jesus have to die in order for us to be forgiven?
•Hell