How do we know Jesus saved his people from their sins?

In Matthew 1:21, the angel tells Joseph that Mary will bear a son, and 'thou shalt call his name Jesus: for he shall save his people from their sins.' This statement affirms the entire purpose of Jesus’ life and death. Unlike other births, Christ was born to die, fulfilling the righteous requirements of the law on our behalf. All of Christ's works, from His perfect life to His sacrificial death, are directed toward the salvation of His people. His work of redemption is completed in His death at Calvary, where He took upon Himself the sins of His people, ensuring that His atoning sacrifice brings forgiveness and reconciliation to all who believe in Him.