You can translate this in an active form. Alternate translation: "For in times past, the prophets wrote everything in the Scriptures to teach us" (See: [[rc://en/ta/man/translate/figs-activepassive]])
Here "have confidence" means that the believers will know that God will fulfill his promises. You can make explicit the full meaning in your translation. Alternate translation: "In this way the scriptures will encourage us to expect that God will do for us everything that he has promised" (See: [[rc://en/ta/man/translate/figs-explicit]])