The answer is: Isaiah

Isa 40:3

In the Gospel of Mark, quoting Isaiah right at the beginning sets the stage for Jesus' ministry and helps establish a rich thematic link between Old Testament prophecy and the arrival of the Messiah. When Mark cites Isaiah 40:3, he connects Jesus’s path to the long-awaited fulfillment of God’s promises. Isaiah’s words call for a voice crying out in the wilderness, symbolizing preparation and anticipation—perfectly mirroring John the Baptist's pivotal role in heralding Jesus' coming.

Isn’t it fascinating how Mark takes a significant prophectic quote that references spiritual preparation? The imagery of making paths straight symbolizes not only physical paths but also the spiritual readiness of hearts for Jesus' teachings. And did you know that marking this closing of Old Testament prophecy with its opening is a literary technique that continues throughout the New Testament? This prophetic echo even resonates in its thematic core, harmonizing stories of both John and Jesus as embodiments of hope and transformative power.

Also interesting to note is the historical context of Isaiah, being written during a time of upheaval for the Israelites. They yearned for liberation and restoration, showing how profound prophecies shaped expectations for future saviors through the ages. It’s a reminder that such texts often speak vividly across time, reflecting both the struggles and hopeful anticipation of many communities seeking meaning and direction.