It seems possible that the way the concept of sin is used has, often, become a distraction from the message of the gospel. This is not to say that sin should be downplayed, or denied, or anything like that. Rather, the idea here is that there is a way to address sin that is clearly legalism, and a way that is a more subtle form of legalism, and then there is the way Christ seems to view it.
"Not everyone who says to Me, 'Lord, Lord,' will enter the kingdom of heaven, but he who does the will of My Father who is in heaven will enter. Many will say to Me on that day, 'Lord, Lord, did we not prophesy in Your name, and in Your name cast out demons, and in Your name perform many miracles?' And then I will declare to them, 'I never knew you; DEPART FROM ME, YOU WHO PRACTICE LAWLESSNESS.'
Matthew 7:21-23 (NASB)
"He who believes in Him is not judged; he who does not believe has been judged already, because he has not believed in the name of the only begotten Son of God. This is the judgment, that the Light has come into the world, and men loved the darkness rather than the Light, for their deeds were evil. For everyone who does evil hates the Light, and does not come to the Light for fear that his deeds will be exposed. But he who practices the truth comes to the Light, so that his deeds may be manifested as having been wrought in God."
John 3:18-21 (NASB)
Notice here what Jesus is saying about final judgment. While we are condemned because of sin, everyone who goes to Hell goes for the same reason: rejection of Christ. Note that in Matthew 7, above, Christ says that those who fail to do the will of the Father will not enter Heaven, and then goes on to describe their condemnation as "I never knew you." Which must mean that these two things are the same. Similarly, in John 3, Jesus says plainly that the judgment of those condemned is that "men loved the darkness rather than the Light," in a book that opened by proclaiming that Christ is the Light. This is repeated in Revelation, when the final judgment is being described. Here, people are judged according to their works, but the sole determination for whether or not they are thrown into the lake of fire is whether or not they are recorded in the book of life.
Then I saw a great white throne and Him who sat upon it, from whose presence earth and heaven fled away, and no place was found for them. And I saw the dead, the great and the small, standing before the throne, and books were opened; and another book was opened, which is the book of life; and the dead were judged from the things which were written in the books, according to their deeds. And the sea gave up the dead which were in it, and death and Hades gave up the dead which were in them; and they were judged, every one of them according to their deeds. Then death and Hades were thrown into the lake of fire. This is the second death, the lake of fire. And if anyone's name was not found written in the book of life, he was thrown into the lake of fire.
Revelation 20:11-15 (NASB)
As has been described on here before, the definition of sin rests on God's commandments. But John sums up God's command as being centered on Christ and the outflow of His love for us rather than our works.
Beloved, if our heart does not condemn us, we have confidence before God; and whatever we ask we receive from Him, because we keep His commandments and do the things that are pleasing in His sight. This is His commandment, that we believe in the name of His Son Jesus Christ, and love one another, just as He commanded us.
1 John 3:21-23 (NASB)
Finally, in Matthew, Christ is recorded explaining that all judgment comes back to our treatment of Himself, as expressed by our treatment of others.
"Then He will also say to those on His left, 'Depart from Me, accursed ones, into the eternal fire which has been prepared for the devil and his angels; for I was hungry, and you gave Me nothing to eat; I was thirsty, and you gave Me nothing to drink; I was a stranger, and you did not invite Me in; naked, and you did not clothe Me; sick, and in prison, and you did not visit Me.' Then they themselves also will answer, 'Lord, when did we see You hungry, or thirsty, or a stranger, or naked, or sick, or in prison, and did not take care of You?' Then He will answer them, 'Truly I say to you, to the extent that you did not do it to one of the least of these, you did not do it to Me.' These will go away into eternal punishment, but the righteous into eternal life."
Matthew 25:41-46 (NASB)
A basic understanding of this is what enables David to cry out, after his affair with Bathsheba:
Against You, You only, I have sinned
And done what is evil in Your sight,
So that You are justified when You speak
And blameless when You judge.
Psalm 51:4 (NASB)
Despite the fact that, given the scope of the situation, it's hard to think of anyone David didn't sin against.
What this is all leading to is this: while we are inclined to describe the matter as though people will go to Hell for this sin or that sin, ultimately, the one reason people go to Hell is rejection of Christ in favor of something else.
Consider it this way. If there is something that is used as a standard to determine whether or not Christ is worth giving oneself to, that thing is more important. We'll not address here whether or not any or all of these things are, themselves, sin, but some examples may prove useful. If one is hesitant to submit to Christ because they are unwilling to yield some aspect of their lifestyle, be it hobbies or addictions or sexuality or extra sleep on Sunday morning, then the issue at hand isn't whether or not that thing is sinful, but the fact that it has been placed on a higher priority than Christ. Remember the surprised damned in Matthew 7 above? How many there will be who find themselves on the wrong side of judgment because they put religious practice or biblical law above Christ Himself!
This should impact our lives in two ways. One, we should repeatedly reevaluate our lives and ensure that we are seeking Christ rather than putting anything, including our service to Christ, as a higher priority than the Lord Himself. Secondly, we should reconsider how we discuss sin with the world. We are so eager to defend Biblical views of various sins and explain that people need saved from them, and that's great, but let's not get distracted from the fact that the actual sentence hanging over their head is "I never knew you." We are not out to fix sins - we are on mission to introduce the world to Christ. Let our hearts always be tuned to this goal.