“No weapon that is formed against you will prosper; and every tongue that accuses you in judgment you will condemn. This is the heritage of the servants of the Lord, and their vindication is from Me,” declares the Lord. – Isaiah 54:17

This verse might be interpreted by some to imply that a child of God cannot be physically defeated, oppressed, persecuted, or harmed; however, history quite clearly proves the opposite. For centuries some of God’s choicest servants have suffered man’s wrath and unimaginable horrors. So, what does it mean?
Obviously, the “judgment” of which Isaiah speaks is eternal and the “weapons” are spiritual. Remember that the believer wrestles not against flesh and blood, but principalities, powers, and rulers in dark places (Eph. 6:12). Our enemy, Satan, prowls about like a roaring lion seeking whom he may devour (1 Pet. 5:8).
The only eternal protection a Christ-follower needs is Christ Himself. The righteousness we obtain from the Savior ensures a “not guilty” verdict in the eternal court of heaven. This is the only safety and security we are guaranteed and the only assurance we truly need.
Our peace and confidence are to be found not in our empirical holiness, not in our progress toward perfection, but in the alien righteousness of Jesus Christ that covers our sinfulness and makes us acceptable before a holy God. – Donald Bloesch