We know we need a double cure because we have both the guilt of sin and the need for righteousness.
The necessity of the double cure comes from understanding our sinfulness. Romans 6:23 states that the wages of sin is death, indicating our guilt and the punishment we deserve. Furthermore, being made righteous is not just about being forgiven; it involves being transformed and sanctified. The metaphor of a wicked man facing death shows that simply being pardoned from sin is insufficient if one is not also given the necessary righteousness to stand before God. Thus, we require both the cleansing from the guilt of our sins (the blood) and the impartation of righteousness (the water) in order to be fully redeemed and restored to fellowship with God.
Commentary