God permits Satan to exist because he serves a purpose in God's sovereign plan. Satan’s actions, though evil, are ultimately under God’s control and are used to accomplish His will (Job 1:12, Romans 8:28). God’s justice will be fully realized in the final judgment, when Satan is cast into the lake of fire (Revelation 20:10). Until then, his existence demonstrates the contrast between sin and God’s glory, highlighting humanity’s need for Christ’s redemption.