The answer is: Untie his sandals

Mark 1:7 - Which was work of servants and so he was showing great humility, saying he was not worthy to be a servant of the man who should come after him.

When John the Baptist proclaimed that he wasn't worthy to untie Jesus' sandals, he showcased deep humility and recognition of Jesus' greater role. In the culture of the time, washing someone's feet or even taking off their sandals was reserved for the lowest of servants. By admitting that he couldn't even perform such a basic act for Jesus, John positioned himself not just as a servant but as a humble forerunner, greatly anticipating the arrival of someone far superior.

This echoes the remarkable humility shown throughout the biblical narratives, like when King David danced with abandon before the Lord, casting aside royal dignity. Moreover, in the Gospel of Matthew, Jesus also emphasizes the significance of humility when He tells His followers that unless they become like children, they will not enter the kingdom of heaven.

Interestingly, John's role finds resonance in prophetic literature, often signaling a powerful messenger in Jewish culture, akin to Elijah—who was prophesied to come before the Messiah (Malachi 4:5). It’s these threads of humility, servant leadership, and prophetic preparation that weave together an enduring lesson on the importance of recognizing one's position in relation to the divine.

Ultimately, John's declaration reminds us of the call to approach life with humility and a readiness to herald the greatness of others, especially those entrusted with profound spiritual messages and leadership—the heart of unity across various traditions we might find today.