How do you equate Christian teachings with certain worldly(non Christian) achievements/attitudes?

This question, as worded, might might not be best for this site, but I see an answerable question that can be answered by some of the most fundamental Christian thinking.

The question is essentially, "Why do Christians insist that everyone is a sinner even though a large portion of people are decent people?"

It is clear through out the Bible that everyone, even the most righteous, except Jesus Christ, were sinners as much as any other person, whether believers or not. Paul even plainly states it for us in Romans:

There is no difference between Jew and Gentile, for all have sinned and fall short of the glory of God.
Romans 3:23 (NIV)

"But they are good people. What difference does it make?"

The difference is in the standards between two parties. Not that Christians have better standards, practically speaking, or that they have a better understanding of morality than anyone else. The difference is even more fundamental than that. It is the difference in standards between all mankind and God Almighty. In Isaiah we read:

“For my thoughts are not your thoughts,
  neither are your ways my ways,”
    declares the Lord.
Isaiah 55:8 (NIV)

God has a righteousness that we can only hope to understand (Luke 18:19). It one of the reasons Christians praise Him. The Bible is filled with praises about His righteousness. We just cannot compare to His standards, even when we are doing our best.

Further, Genesis tells us that God made everything (the method being irrelevant to this point) and that when He was done, it was very good. God made everything and it was all good. James also tells us that every good thing comes from God:

Every good and perfect gift is from above, coming down from the Father of the heavenly lights, who does not change like shifting shadows.
James 1:17 (NIV)

So, confidently, Christians insist that all mankind, past and present, are sinners.

To continue in the same thought, the question is now "Why do Christians think they go to Heaven?"

The same verses in Romans 3 plainly tell us (emphasis added):

But now apart from the law the righteousness of God has been made known, to which the Law and the Prophets testify. This righteousness is given through faith in Jesus Christ to all who believe. There is no difference between Jew and Gentile, for all have sinned and fall short of the glory of God, and all are justified freely by his grace through the redemption that came by Christ Jesus. God presented Christ as a sacrifice of atonement, through the shedding of his blood—to be received by faith. He did this to demonstrate his righteousness, because in his forbearance he had left the sins committed beforehand unpunished— he did it to demonstrate his righteousness at the present time, so as to be just and the one who justifies those who have faith in Jesus.

God demonstrated his righteousness, His justice and judgement on all mankind, by sending Christ to the cross. Accepting that Jesus paid the price for your transgression is the central message of salvation that spares you from Hellfire.

God is truly merciful, as we see in the same verses in Isaiah 55 (emphasis added):

Seek the Lord while he may be found;
    call on him while he is near.
  Let the wicked forsake their ways
    and the unrighteous their thoughts.
  Let them [the wicked] turn to the Lord, and he will have mercy on them,
    and to our God, for he will freely pardon.

Repent and turn from your evil ways and God will give you mercy. Accept the free gift of God, Christ's death on the cross, which pays your sin debt, and God will give you grace, eternal life in heaven (Romans 6:23).

This is the fundamental message that all Christians bring to the world.