The Apostle Paul summarizes the Gospel in this way: "Now I would remind you, brothers, of the gospel I preached to you, which you received, in which you stand, and by which you are being saved, if you hold fast to the word I preached to you—unless you believed in vain. For I delivered to you as of first importance what I also received: that Christ died for our sins in accordance with the Scriptures, that he was buried, that he was raised on the third day in accordance with the Scriptures, and that he appeared to Cephas, then to the twelve" (I Corinthians 15:1-5).
All religions purport to offer some message of "good news"! There are those who would tell you the "good news" comes through penance and confession. For others, the "good news" comes through trying to be a good person, keeping certain commandments, and/or following the 'golden rule'. Still others think of the "good news" as a message of socio-economic freedom from political oppression. Unfortunately, all of these attempts to offer "good news" turn out to be "bad news", precisely because these 'gospels' are all offered on the basis of our good works.
But the truly "good news" of the Gospel that we find presented in the Bible is different. The Apostle Paul highlights a number of in these few short verses that set this 'Gospel' message apart.
(1) The Gospel is not an 'abstract philosophy' but rooted in history.
All religions at some level have elements of 'faith'; hence the reason why even secular society can speak about a "plurality of religions and faiths"! Even the person who claims to not believe in anything or claims to have 'faith' in nothing inconsistently ends up putting 'faith' in his or her agnosticism or atheism.
What sets Christianity apart? It's that our faith is directed to a Jesus that "died...was buried...and raised on the third day." That is to say, our claims about the Gospel message are historically verifiable This is why the first four books of the New Testament are often called 'the 4 Gospels' -- that is, they bear historical testimony to the life, death, resurrection, and ascension of Jesus Christ.
(2) The Gospel speaks to many issues, but at the center of them all is our sin.
People in all walks of life experience pain and anguish. Individuals lose their jobs; marriages come to an end and families get ripped apart through adultery; children are abused by their parents; genocide takes places in countries with political instability; and on the list could go. Christianity recognizes that all of these things happen because of sin, and most people have no problem speaking about 'sin' in this way because it provides a category for them to explain the truly wicked things that happen in the world. As long as the problem of sin is seen as external ("out there in the world") then it's not a particularly hard concept to grasp.
But Christianity doesn't simply believe that doing bad things makes one wicked. Christianity teaches that we do bad things because we ourselves are wicked. Our personal deeds (good or bad) don't just randomly happen in a vacuum; they flow out of a predisposition to do one thing and not another thing. And this is not true for just certain class of really wicked people -- the Bible makes clear that it applies to us all. God's verdict of the world's status is that "all have sinned and fallen short of the glory of God" (Romans 3:23).
This is what Paul means here by "our sin". It's one thing to blame others or the world for 'their sin'; it's quite another thing to confess your own sin! But it was for this very reason that Jesus came to die. He was not merely giving us a good example for how to live; if that was all Jesus was trying to do, then his 'dying' is not a very good example. The reason Jesus had to die is because "our sin" doesn't simply alienate ourselves from each other but ultimately alienates us from God.
(3) There are many things in life that are important, but believing the Gospel is of "first importance".
People often come to the Bible and/or Church looking for answers to life's difficulties and troubles but sometimes with our own idea of what we think are important. The wonderfully refreshing and yet at the same time challenging thing about Christianity is that it does not skirt away from addressing such difficulties but rather that it addresses them with a seriousness and directness that we often underestimate. Our problem is not simply that we are in need of a moral boost to live better or that we simply need a 'yearly tune up' to get our life priorities back in line. Our problem is infinitely greater because we are infinitely guilty.
But the 'good news' (the Gospel) of Christianity is summed up in the resurrection of Jesus. And this message is so important that the apostle Paul can go on to say that "if Christ has not been raised, your faith is futile and you are still in your sins" (I Cor. 15:17)! Someone has said this is Christianity's 'line in the sand' -- the one event on which the entirety of the Christian message (and even the Bible itself) comes down to. Jesus had to do something radical to rescue a world plunged in sin and darkness. For this reason, the resurrection gets to the heart of the "importance" of the Bible's message. The resurrection is more than simply an event the Church celebrates once a year at Easter time; rather, the resurrection is God's direct answer to the core problem of our personal sin and guilt.