CLAIM: Mark cites from Malachi 3:1 and Isaiah 40:3. Why does he do this?
RESPONSE: Mark quoted Malachi and Isaiah in order to establish Jesus’ deity.
Malachi predicts that a messenger would come before God when he arrives on Earth. Malachi also says that God himself will come to his Temple. Thus this is a prophecy that can be falsified. If the Temple is destroyed before the Lord comes, then this prophecy would be false. But since Jesus appears in the Temple (and he is fully God), then he is the only person who could have fulfilled this prophecy. A few decades after Jesus dies (AD 70), the Temple was destroyed.
Isaiah predicts that a voice would prepare the way for Yahweh God (LORD) from the wilderness. This is fulfilled by John the Baptist in verse 4, where he teaches about how to have your sins forgiven.