The answer is: Isaiah

Isa 40:3

The Gospel of Mark opens with a profound proclamation rooted in the words of Isaiah, underscoring the deep connection between the Old and New Testaments. When you delve into Mark 1:2-3, he introduces John the Baptist as a forerunner to Jesus, echoing Isaiah 40:3, "A voice crying in the wilderness: Prepare the way for the Lord." This sets the tone for the entire Gospel; it's like a divine tear in time, linking centuries of prophetic tradition directly to the moment of Jesus' arrival.

Isaiah, often referred to as the "fifth gospel," emphasizes themes of hope and redemption. Interestingly, the prophetic landscape of the Old Testament showcases the often-frustrating struggle of Israel’s spiritual renaissance, prompting the question, 'Could it be different this time?' That resonance seems present each time Isaiah is quoted.

John the Baptist’s move from the wilderness is also symbolic—it's a compelling call to seek something both ancient and entirely new. His location signifies a break from the norms. Fun trivia: According to tradition, the locusts and honey John the Baptist ate were not just symbolic of simplicity; they hinted at a profound rejection of luxury and indulgence!

So, when reading Mark, you see how that initial quote catalyzes a movement. It’s about preparation and transformation—not just of roads, but of hearts craving authenticity in old prophecies, preparing for a revolutionary message that would echo through time.