Seriously, you think mixed dancing, swimming qualifies as a sin? A transgression that deserves to be punished by eternal damnation? That godly perfection is a state of being that would never, ever include such "immorality"?
(And no, the Sermon of the Mount does not state this.)
Since so many Christians on this thread apparently thought that I'm unduly fixated on a very literal, very abhorrent conception of hell, here's what *I* think the "good news" is.
Most religions in the ancient world revolved around the concept of sacrifice, of killing livestock (or, in more extreme cases, even humans) in order to appease uncanny deities that were believed to exact gruesome revenge on any mortal who might deliberately or unknowingly cause offense. In a world that did not understand the naturalistic causes of plague, floods, earthquakes or lightning strikes, angry deities were the catch-all explanation for bad things happening, and moral transgressions (to be redeemed by sacrifice) were the reason behind (respectively the remedy for) such calamities.
Proto-Judaism had already taken some steps towards a more pragmatic approach, first outlawing human sacrifice by means of a tale that emphasized it was the attitude behind the act that mattered most (in Abraham's tale), and then substituting animals for those transgressions that people were not aware of committing.
But Christianity made an end to the logic of sacrifice its core doctrine: the "good news" at its heart is that its god loved people SO MUCH that he turned himself into a sacrifice, setting the captives free and urging his "Father" to forgive those who unknowingly committed such a grave transgression. God does not want your blood, or that of your relatives. He doesn't even want you to slaughter some animals for repentance, for you ARE forgiven. The truly good news is that God LOVES the world, instead of being an angry, unfathomable force bent on receiving protection money in the form of blood.
THAT is what set Christianity apart from other religions of late antiquity, that - more so than its apocalyptic promise of a World To Come - is at the core of its overwhelming success. The "Prince of Peace" set the captives free and urged us to love even our enemies.
Yes, there are verses that show Jesus condemning those who do not help the helpless, feed the hungry, or clothe the destitute in the harshest of terms, stating that such people have no place in the world to come, and belong on the burning rubbish heap in the valley of Hinnom. And he also states that those who cry "Lord, Lord", prophesying in his name and even performing miracles are not necessarily exalted if they fail to show love and compassion for the unfortunate.
But that is not the "good news" - it's an expression of a moral code that demands something more than saying the right words or waving the right membership card. Jesus was fairly radical (in a good way), and his ethos is almost diametrically opposed to the nationalist consumer-capitalism embraced by conservative Christianity today.