In Christian theology, Jesus is sometimes referred to as a Redeemer. This refers to the salvation he is believed to have accomplished, and is based on the metaphor of redemption, or "buying back". Although the Gospels do not use the title "Redeemer", the word "redemption" is used in several of Paul's letters. Leon Morris says that "Paul uses the concept of redemption primarily to speak of the saving significance of the death of Christ." The English word redemption means 'repurchase' or 'buy back', and in the Old Testament referred to the ransom of slaves (Exodus 21:8). In the New Testament the redemption word group is used to refer both to deliverance from sin and freedom from captivity.