The church is not a great nation, military power, or cultural institution organized to change this world. We exist solely to grow, overcome, and glorify God primarily through the witness of our lives lived in preparing for God's Kingdom.
The illustration Isaiah uses portrays us on trial and standing before a court of law. The primary witness is one's life. Even so, some of God's children are specifically chosen to witness through their words as well as through their lives. Each believer is a witness before the world of the worth of His Lord, Jesus Christ, and His purpose. Those living by faith make the witness.
How can one witness well unless he knows what to do? How can he know what to do unless he is taught? This is a major purpose of the Sabbath commandment. God established it to provide a means of unified instruction, and it is therefore a major player in the process of conversion and witnessing for Him.