"I baptize with water, but there stands One among you whom you do not know. It is he who, coming after me, is preferred before me, whose sandal strap I am not worthy to loose." (John 1:26-27)
John's mention of Jesus' sandal strap was an idiom, an expression of the Jews. A disciple of a rabbi, such as Jesus' disciples, not only attended the lectures of the rabbi and learned the lessons that he taught, he took on the role of a servant. The disciple actually functioned as the personal slave of the rabbi and took care of all of his needs - making his housing arrangements, getting his food, and so forth. We see examples of this in the ministry of Jesus, such as the occasion when he sent his disciples into Jerusalem to make sure that a room was reserved where he could celebrate the Passover. But the one thing that differentiated a disciples in a rabbinical school from an actual bondslave was that the disciples was never required to take care of the shoes or the sandals of his teacher. A slave could be reduced to that humiliating task, but not a disciple. Therefore, when John said, "I am not even worthy to unstrap his sandals," he was saying: "Don't look at me. I'm lower than a disciple. I'm even lower than a slave. I'm not even worthy to untie his shoes, to take off his sandals, to clean his feet. Don't look to me. Look to him."
I believe John put this incident right at the beginning of his Gospel to help focus his readers' attention on the One whom John's Gospel announces. John is saying, through John the Baptist, "It's time to make straight the highway of our God." (R.C. Sproul, John)
Grace upon grace,