"This is because he will be great." Zechariah and the "many" will rejoice because John will be "great in the sight of the Lord." The rest of verse 15 tells how God wants John to live.
This can be stated in active form. Alternate translation: "the Holy Spirit will empower him" or "the Holy Spirit will guide him" Make sure it does not sound similar to what an evil spirit might do to a person. (See: [[rc://en/ta/man/translate/figs-activepassive]])