John's answer about a greater person coming clearly implies that John is not the Christ. It may be helpful to state this clearly for your audience. AT: "John clarified that he was not the Christ by saying to them all" (See: [[rc://en/ta/man/translate/figs-explicit]])
"not important enough even to loosen the straps of his sandals." Untying the straps of sandals was a duty of a slave. John was saying that the one who would come is so great that John was not even worthy enough to be his slave.
This metaphor compares literal baptism that brings a person into contact with water to a spiritual baptism that brings them into contact with the Holy Spirit and with fire. (See: [[rc://en/ta/man/translate/figs-metaphor]])
Here the word "fire" may refer to 1) judgment or 2) purification. It is preferred to leave it as "fire" (See: [[rc://en/ta/man/translate/figs-metaphor]])