How do we know that Christ's sacrifice for sin is effective?

Christ's effectiveness as a high priest is evidenced in Hebrews 2:17, where it states he made reconciliation for the sins of his people. His sacrifice blotted out sins completely, not just covering them temporarily as with the Old Testament sacrifices. As fully God and fully man, Christ was able to bear the weight of sin and satisfy the justice of God, ensuring that those he represents are declared righteous and justified (2 Corinthians 5:21). Thus, his sacrifice is sufficient and permanent, giving assurance to believers.