The answer is: Isaiah

Isa 40:3

In the Gospel of Mark, the opening lines not only set the tone for Christ's ministry but also link it back to the profound prophecies of Isaiah—a heavyweight in the world of Hebrew scriptures. By quoting Isaiah, Mark isn't simply giving us a historical tidbit; he's packaging Jesus' mission in the rich prophecies that foretold the coming of a forerunner who would prepare the way for the Lord. That's quite significant!

Isaiah 40:3 proclaims, "A voice of one calling in the wilderness: 'Prepare the way for the Lord, make straight paths for him.'" This intriguing imagery of "the wilderness" represents not just a physical location but a spiritual state, where transformation and renewal occur. Just as John the Baptist emerges from the wilderness to herald Jesus’ arrival, it signals a shift from darkness to fulfillment.

Here's a fun fact: The earliest part of Isaiah, known as First Isaiah (Chapters 1-39), deals with various themes such as judgment, hope, and comfort for the people. Rather than being merely prophetic, it reflects the social and political unrest of its time. Outrageous fears, divine warnings, and grand illustrations tied the hopes of the Israelites directly to the prophetic figures like Isaiah himself, making the New Testament setting feel even more electrifying to the audience hearing it read for the first time!

Thus, engaging with Mark highlights how foundational the prophetic tradition is to understanding the mission of Jesus—where Old Testament hope and New Testament action collide beautifully.