What is Mark’s “good news” or Gospel?
With his introduction, “The beginning of the good news of Jesus Christ, the Son of God” (1:1), Mark is ascribing divine authority to his gospel: this is not merely the evangelist’s story about Jesus; this is Jesus’ own word; it is his good news to his people. Mark then defines what the "good news" is by quoting the prophets Malachi and Isaiah. By identifying the fore-running "messenger" of Malachi 3:1, and the “crying voice” of Isaiah 40:3 as John the Baptist (cf. 1:1-4), Mark is boldly proclaiming that Jesus is the Messiah promised to Israel, which she knew according to her prophets would be God. Also from the context of Isaiah 40 it is further understood that Mark is announcing, "the glory (i.e., the salvation, cf. Isaiah 52:10; Luke 3:6) of God is about to be revealed" (cf. Isaiah 40:5).
So what is this good news? What is this promised salvation? At the conclusion of Mark's prologue, he defines the "good news" as follows: 1) the time is fulfilled, and 2) the kingdom of God has come near (1:14,15). We need look no further then to again, Isaiah 40, to see this promise about to be fulfilled by Jesus Christ defined as the comfort of God’s people, through the forgiveness of their sins: "Comfort, O comfort my people, says your God. Speak tenderly to Jerusalem, and cry to her that she has served her term, that her penalty is paid, that she has received from the Lord's hand double for all her sins" (Isaiah 40:1-2). At the conclusion of the gospel, included in the “shorter ending” following 16:8, the good news is equated with the proclamation of “eternal salvation,” and this is indeed theologically consistent with Mark’s opening quotation from Isaiah.
 Malachi 3:1 See, I am sending my messenger to prepare the way before me, and the Lord whom you seek will suddenly come to his temple. The messenger of the covenant in whom you delight—indeed, he is coming, says the Lord of hosts."
 Isaiah 40:3 A voice cries out: "In the wilderness prepare the way of the Lord, make straight in the desert a highway for our God.
 The earliest manuscripts do not contain Mark 16:9-20. The scholarship community is divided over whether this “longer ending” was part of Mark’s original gospel. Some manuscripts contain a “shorter ending” following verse 8, which reads: “And all that had been commanded they told briefly to those around Peter. And afterward Jesus himself sent out through them, from east to west, the sacred and imperishable proclamation of eternal salvation.” For more on the various perspectives on the ending of Mark, please see: Maurice Robinson et al., Perspectives On the Ending of Mark: Four Views, ed. David Alan Black (Nashville, Tenn.: B&H Academic, 2008).