Why must a substitute be sinless?

The necessity for a sinless substitute arises from the nature of sin and the holiness of God. The sacrificial system in the Old Testament required that offerings be without blemish, representing purity and the requirement for a perfect sacrifice to atone for sin. Jesus, being without sin, is the only One who could justly take upon Himself the sins of mankind. The Apostle Peter emphasizes this in 1 Peter 1:19, stating that we were redeemed 'with the precious blood of Christ, as of a lamb without blemish and without spot.' This sinless nature ensures that the sacrifice is fully acceptable to God, thereby securing our justification and reconciliation through His righteousness rather than our own.