It's easy as one raised in the church, and surrounded from earliest memory by the gospel of salvation from sin through faith in Jesus, to assume too much knowledge of, and engagement with these "basics of Christianity" on the part of those not raised in that environment.
The predominant worldview today is materialism (meaning that reality is understood through the lens that all of "life, the universe, and everything" is understood only by reference to physical Law, and without reference to supernatural causes). In this worldview, there's no such thing as an absolute moral/ethical standard -- codes of morality are just individual or social constructs which have in some way evolved as useful to the survival of tribe/species. Said more concisely, a materialist worldview leads to a relativist view of morals.
Even when people aren't themselves ardent materialists/relativists, their thinking has often been deeply colored by some of the assumptions of this worldview, with results that are very damaging to a proper understanding of the core truths of Christianity. In order to communicate the gospel effectively we have to begin by properly defining some of these foundational concepts that don't even exist in the materialist/relativist environment that so many have been raised in.
A good example of this is with regard to the definition of sin. The notion of sin is, of course, absolutely central to the orthodox Christian worldview. But the acceptance of a relativist ethic makes the gospel of Christ meaningless.
To make this concrete, ask a random sampling teenagers and young adults what "sin" is. The first answer is usually kind of vague, although technically correct: sin is "doing something wrong". If you probe a little by asking "wrong according to who?" the answer will often take one of two flavors: (a) wrong according to them, or (b) wrong according to a social group of which the person is a part.
If a person has this view of sin, what does it mean to them when someone says to them that Jesus came to "forgive them" or to "save them from their sins"?
If their "sin" was the violation of their own moral code, then "salvation" only means release from whatever feeling of guilt they may have about having broken their own rules.
On the other hand, if their "sin" was breaking the moral code of their tribe or social group, then "salvation" would have to be construed as somehow "making right" the broken relationship between the "sinner" and their offended tribe.
In either of these cases, it's easy to see why people would be confused about the gospel. In both cases there doesn't seem to be any connection between the death of Christ and the "salvation" effected. That is, why would the fact that Jesus died 2000 years ago make me more likely to forgive myself for breaking my own rules? And what relevance would His death have to whether I've "made up" to my social group whatever the penalty was for my breaking of their rules? The answer is, of course, nothing.
So, the gospel of salvation from sin through Christ can only make sense when sin is understood to be a human being's lack of obedience to an absolute Law of God. The only one who can forgive a debt is the one to whom it is owed. The only one who can forgive a sin is the one who was sinned against. Said another way, the only way that God can forgive all sin is if all sin is ultimately against God! To the extent that we deny the universality or applicability of the Law or our obligation to God for breaking it, the Christian gospel is meaningless.
On the other hand, it is only when a person knows the nature of sin, and when they understand Who they have sinned against, and what the consequence of that sin is, that they can ask for the grace of forgiveness that is given through the gift of faith in Christ.
So, why are Christians so "obsessed" with sin? First, we're at war with sin because we've been taught to love God and to hate the things that God has said grieve Him. Second, because we treasure the enjoyment of friendship with Him and hate the things that hinder that enjoyment. Third, because we have been made conscious of what Jesus suffered in order to save us from our sin, and our gratitude to Him makes us desperately want to avoid sinning further. Fourth, because the Bible teaches that the entire universe has been made subject to decay as a result of sin -- that is, all of the death and evil in the world are ultimately attributable to rebellion against God. Fifth, because we know that sin is deceptive, and that there are many false Christians throughout history who started out believing that they were Christians but subsequently fell away because they were gradually led from their faith by compromise in little sins.
Of course, it's important to emphasize that the battle against sin is not a battle to "earn our salvation" or to root out sin in others, but rather to seek through the Spirit to put to death our sinful nature (Rom 8:13), to encourage our brothers and sisters "to press on to the prize of the upward calling of God in Christ Jesus" (Phil 3:14), and to pray for people who don't yet know the Lord that God would "grant them repentance leading to the knowledge of the truth, and they may come to their senses and escape from the snare of the devil, having been held captive by him to do his will" (2 Tim 4:25-26)
I should acknowledge that some people, having read the above, might concede that the Christian gospel is incompatible with a relativistic ethical framework, but would rather jettison the gospel than the relativistic framework. More in response to that issue in a future post ...
We are to go to Jesus outside the camp. Why? 
