In Isaiah, chapter 40, we have these, rather pointed verses:

2 Speak tenderly to Jerusalem,
and proclaim to her
that her hard service has been completed,
that her sin has been paid for,
that she has received from the Lord’s hand
double for all her sins.

3 A voice of one calling:
“In the wilderness prepare
the way for the Lord;1
make straight in the desert
a highway for our God.2

4 Every valley shall be raised up,
every mountain and hill made low;
the rough ground shall become level,
the rugged places a plain.

5 And the glory of the Lord will be revealed,
and all people will see it together.
For the mouth of the Lord has spoken.”

Isaiah 40 : 2-5

And so in these two short passages, the author of Mark has woven together two larger passages in the Old Testament that speak not only of a messenger, but also of a Lord of Armies, and a great sacrifice being made of Jerusalem. Any contemporary Jew would have been familiar with the larger, connected texts, but those unfamiliar with the older writings would not understand the greater context.

The verses from Malichai suggest human sacrifice by fire would be the fate of Jerusalem. The words from Isaiah seem to reflect the actions of the Imperial legions throwing Jerusalem’s great temple into the valley below, thus making flat what had been famously hilly. The violent and devastating actions of the Flavians are clearly reflected in the larger selections from which these quotes are drawn. This is an example of how a great deal of meaning has been constructed out of a small, choice collection of words. 

This is all done as a matter of introduction to the character of John the Baptist in verse 4, who is identified as the ‘voice of the one crying the wilderness’, and to explain his purpose in the story in the context of Hebrew prophetic writings. He is an agent of Caesar, here to “make ready the road” for Caesar. And we know, as did the first readers of Mark, Caesar did empty his wrath upon the Judeans, and that such a prophecy was no idle threat.