Who said, “See, I am sending my messenger ahead of you, who will prepare your way”?

The gospel of Mark begins with a prophecy, “See, I am sending my messenger ahead of you, who will prepare your way; the voice of one crying out in the wilderness: ‘Prepare the way of the Lord, make his paths straight’”. Mark uses this prophecy to introduce John the Baptist. But where does it come from?

Mark attributes the prophecy to Isaiah:

“As it is written in the prophet Isaiah, ‘See, I am sending my messenger ahead of you, who will prepare your way; the voice of one crying out in the wilderness: “Prepare the way of the Lord, make his paths straight”‘. [Mark 1:2-3, NRSV]

If we look at Isaiah, however, we see that only the second half of the prophecy is present:

“A voice cries out: ‘In the wilderness prepare the way of the Lord, make straight in the desert a highway for our God.’” [Isaiah 40:3, NRSV]

For the first part of the prophecy, we must turn to either Exodus or Malachi:

“I am going to send an angel in front of you, to guard you on the way and to bring you to the place that I have prepared.” [Exodus 23:20, NRSV]

“See, I am sending my messenger to prepare the way before me, and the Lord whom you seek will suddenly come to his temple.” [Malachi 3:1a, NRSV]

Mark’s attribution of the first part of the prophecy to Isaiah is thus an error: it isn’t written in the prophet Isaiah “See, I am sending my messenger ahead of you, who will prepare your way”, but only in Exodus and Malachi.