en_tn_condensed/jhn/12/38.md

so that the word of Isaiah the prophet would be fulfilled

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/jit/figs-activepassive)

Lord, who has believed our report, and to whom has the arm of the Lord been revealed?

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/jit/figs-rquestion)

the arm of the Lord

This is a metonym that refers to the Lord's ability to rescue with power. (See: rc://en/ta/man/jit/figs-metonymy)