You can translate this in an active form. Alternate translation: "in order to fulfill the message of Isaiah the prophet" (See: [[rc://en/ta/man/translate/figs-activepassive]])
This appears in the form of two rhetorical questions to express the prophet's dismay that the people do not believe his message.They may be stated as a single rhetorical question, Alternate translation: "Lord, hardly anyone has believed our message, even though they have seen that you are powerfully able to save them!" (See: [[rc://en/ta/man/translate/figs-rquestion]])