This has quotes within a quote. You could translate the direct quotations as an indirect quotations. Alternate translation: "If we say that we believe John received his authority from heaven, then Jesus will ask us why we did not believe John." (See: [[rc://en/ta/man/translate/figs-quotesinquotes]] and [[rc://en/ta/man/translate/figs-quotations]])
The religious leaders know that Jesus could scold them with this rhetorical question. Alternate translation: "Then you should have believed John the Baptist" (See: [[rc://en/ta/man/translate/figs-rquestion]])