The answer is: Isaiah

Isa 40:3

In the Gospel of Mark, the first shout of the Prophet Isaiah heralds the arrival of something monumental. Mark, known for his brisk narrative style, kicks off his Gospel not with Jesus Himself, but with these prophetic words that set the stage for what is about to unfold. "Prepare the way for the Lord, make straight paths for him" (Isaiah 40:3) resonates as a bridge between the long-echoed promises of God and the immediate coming of John the Baptist, who will prepare the hearts of many for Jesus.

What’s fascinating here is how Isaiah's words pulsate with both Old and New Testament significance. Isaiah 40 brings a message of comfort and hope for a people groaning under exile; they’re about to receive good news. Remarkably, that hope finds its ignition in the clay of human reality through John’s rather rugged ministry in the wilderness—perfectly mismatched with royal processions.

Moreover, the use of prophecy ran like a thread through Jewish consciousness, creating expectation for the authoritative voice of God. This was crucial for both Mark's audience and the coming followers of Jesus, who inherited that prophetic energy alongside a revolutionary figure who wouldn’t fit neatly into their anticipations.

So, with Isaiah's groundwork laid, Mark crafts a narrative spotlighting immediacy—a frenzied urgency—asking readers to be alert for the transformative teachings of Jesus. It's about preparing ourselves for the profound new realities of compassion, justice, and love to dive deep into!