How do we know Christ's atonement was sufficient and effective?

The sufficiency and effectiveness of Christ's atonement rest on the fact that He died specifically for His sheep, as illustrated in John 10:28-29, where He states that no one can snatch them out of His hand. This aligns with the historic Reformed view that Christ's death accomplished salvation for His elect rather than merely making salvation possible for all. The vicarious nature of Christ's sacrifice is emphasized, where He bore the penalty for the sins of His people, ensuring that all for whom He died will undoubtedly be saved. In 2 Corinthians 5:21, we see that God made Him who knew no sin to be sin for us, signifying the nature of His work as truly effectual for salvation.