Paul uses this question to get the attention of his readers. Alternate translation: "This is what we must say" (See: [[rc://en/ta/man/translate/figs-rquestion]])
Here "by faith" refers to placing one's trust in Christ. You can make this explicit in your translation. Alternate translation: "because God made them right with him when they trusted in Christ" (See: [[rc://en/ta/man/translate/figs-explicit]])