The beginning of the gospel of Jesus Christ, the Son of God. As it is written in Isaiah the prophet, "Behold, I send my messenger before your face, who will prepare your way, the voice of one crying in the wilderness:
'Prepare the way of the Lord, make his paths straight,' " John appeared, baptizing in the wilderness and proclaiming a baptism of repentance for the forgiveness of sins. And all the country of Judea and all Jerusalem were going out to him and were being baptized by him in the river Jordan, confessing their sins. Now John was clothed with camel's hair and wore a leather belt around his waist and ate locusts and wild honey.
(Mark 1:1-6 ESV)
The first two verses point directly to the Old Testament. Mark illustrates the relationship between the Old and New Testament, which the church fathers were VERY interested in. Irenaeus especially used these verses to show that the OT God and the NT God were the same -- the prophets, after all, didn't foretell the coming of a new and improved deity. This conflicted with Marcion's teaching that the God of the Old Testament was a vengeful, angry God, but the God of the New Testament was a loving and merciful God of grace.
It is interesting that Mark attributes the prophecy to Isaiah, when it's actually two prophecies in two books.
Malachi 3:1 is the prophecy of the messenger, while Isaiah 40:3 mentions the voice crying in the wilderness. I've heard a lot of different things about this -- that Malachi and Isaiah were on the same scroll, but the scrolls were referred to by the name of the major prophet, that many writers conflate (merge together) prophecies and only credit the major prophet. Of course, some manuscripts attribut the prophecy to "the Prophets", rather than to any specific one. There's a great treatment of this issue here
The thing I got the most out of this passage comes from the description of John. I wondered why he had to look this way, why he had to be a "voice crying in the wilderness". When the Jews saw someone looking like him, who ate very little, had poor clothing, and no "hometown" (he had lived in the wilderness for most of his life). He had no reputation, nobody knew who had taught him. Yet he spoke with the authority of someone who had been with God. They knew there was only one way that John could have survived living the way he did -- God was with him, supplying him with everything he needed. THAT is what gave John his credibility with the people.
I was fascinated at the way the church fathers dealt with John. They saw him as a type of the Law -- clad in filthy rags, just as our righteousness is as filthy rags, sustained by God, meant to point us all toward the Saviour. They pointed especially to John's statement that "He must increase, and I must decrease" to show not only Johns feeling about his ministry after Christ's appearing, but that the Law must diminish while Grace abounded. I think this view, more than any other, shows the true relationship between Grace and Law. The Law shows us how good we really need to be to merit heaven, and shows us that we cannot do it ourselves. That's when Grace comes in, and gives us a Way.