"The beginning of the good news about Jesus the Messiah, the Son of God, as it is written in Isaiah the prophet..."
The gospel according to Mark does not begin at Jesus's birth like in Matthew and Luke, but begins in Isaiah 40. Those familiar with the Old Testament (Mark and potentially his Jewish/Christian audience) know what Isaiah 40 is about from memory. It announces there will be comfort for God's people and that sin has been paid for. (Isaiah 40:1-2) It announces the coming of the LORD and the revelation of His glory and one who will prepare His way for Him. (Is 40:3-5) And though we are but temporary, the Lord is everlasting and his kingdom will have no end. (Is 40:6ff)
Mark is not only saying that the LORD is coming with power to rule with a mighty arm, but in the gentlest of ways as a shepherd tends his flock (Is 40:9-11), but that he is here and his name is Jesus Christ. This God in Christ, who created the world and whom none can fathom (Is 40:12-14) and is incomparable (Is 40:15-26) is now seen and touched and accessible by all. He is here to give strength to the weary and power to the weak so that all who trust in the LORD will renew their strength and soar on wings like eagles. (Isaiah 40:27-31)
And that, my friends, is only the beginning, the beginning of the good news about Jesus the Messiah, the Son of God.