What is the mystery of Christ mentioned in the Bible?

In Colossians 4:3, Paul refers to the mystery of Christ, emphasizing that it encompasses profound truths about God and redemption that were not fully understood before Christ's arrival. This mystery highlights God's plan for salvation through Jesus, wherein He reconciles sinners to Himself. Ephesians 3:6 explains that this mystery also includes that the Gentiles are fellow heirs and partakers of God's promise in Christ through the gospel. Therefore, understanding this mystery leads to a deeper appreciation of God's grace and the expansive nature of His redemptive work across all nations.