People will realize that he is a prophet. This can be stated in active form. AT: "people will know that you are a prophet" (See: [[rc://en/ta/man/translate/figs-activepassive]])
These words are a euphemism for God. AT: "who serves the Most High" or "who speaks for God Most High" (See: [[rc://en/ta/man/translate/figs-euphemism]])
"the face of" someone can be an idiom that refers to the that person's presence. It is sometimes omitted in translation. AT: "the Lord" See how you translated this in [Luke 1:17](./17.md). (See: [[rc://en/ta/man/translate/figs-idiom]])
This is a metaphor that means that John will prepare the people to listen to and believe the Lord's message. (See: [[rc://en/ta/man/translate/figs-metaphor]])