Imagine the burden of the guilt from your sin you felt before you were saved, when you realized the depth of your fallenness, unworthiness, and broken relationship with God, your Father. Imagine the overwhelming realization that you were guilty and you could never please God by your own efforts. That is the burden Martin Luther and many others lived under with no hope. No Gospel - bad news.
Then Luther, as a few had before him - John Hus and John Wycliffe, stumbled upon God's Word and the good news that had not been preached to them: Reconciliation with God was possible through Jesus. Forgiveness. A relationship with God. Gospel - good news.
“For it is by grace you have been saved, through faith—and this not from yourselves, it is the gift of God” (Eph. 2:8
It's hard to imagine living with the burden of sin and never hearing the Gospel. That is the significance 
of the Reformation that was finally sparked on this day in 1517 when Martin Luther nailed the 95 Theses to the church door in Wittenburg hoping to initiate a discussion to reform the church. And it's the Gospel, the good news, that is the legacy of the Reformation that is our blessing each and every day, to live under the Gospel, the good news.
Here's an excellent description of the significance of the Reformation from Here We Stand by Hermann Sasse (pp. 65-71):
It is not, therefore, the return to the Scriptures in itself which makes the Reformation of the sixteenth century one of the great and unique events of church history. The nature of the Reformation must be sought, rather, in the particular kind of return to the Bible….How often has the church been reformed ‘according to God’s Word’!...[T]he…Reformation, in its essential nature, is nothing else than a rediscovery of the gospel….
The rediscovery of the Scriptural truth concerning the justification of the sinner by grace alone, through faith alone, is nothing less than the rediscovery of the Gospel. For, if this truth is forgotten, the Gospel must be interpreted as a system of morals or as a theory of religious metaphysics. Consequently, this discovery constitutes the reformation of the church. It revealed once again that truth by which alone the church lives.
For the church does not live by morals, by the knowledge and observance of God’s law. Nor does it live by religion, by lofty experiences of the divine and an awareness of the mysteries of God. It lives solely by the forgiveness of sins. Hence reformation does not consist, as the late Middle Ages believed, and has even been believed in wide circles of the Protestant world, of an ethico-religious correction, of a moral quickening and a spiritual deepening throughout the church. It consists, rather, according to its own peculiar nature, of the revival of the preaching of the Gospel of the forgiveness of sins for Christ’s sake….
The [Reformation] understanding of the Scriptures, we say, came as a result of this rediscovery of the Gospel. At all events, it has been the conviction of the [Reformation] that the sola scriptura is conditioned by the sola fide, that a real return to the Scriptures was made possible only by a new understanding of the Gospel. It is in this sense that the Apology to the Augsburg Confession speaks of Justification as “the chief topic of Christian doctrine…which is of especial service for the clear, correct understanding of the entire Holy Scriptures, and alone shows the way to the unspeakable treasure and right knowledge of Christ, and alone opens the door to the entire Bible." This view alone guards against the false, legalistic conception of the Bible as a law-book ….[T]he Reformation was a renovation of the church brought about by the rediscovery and renewed proclamation of the pure doctrine of the Gospel of the forgiveness of sins.
And, thank God, our renovation as well.
