What does the Bible mean by the mystery of the gospel?

The mystery of the gospel, as discussed in Romans 11 and Ephesians 3, represents the profound truth that was hidden in ages past but has now been revealed through Jesus Christ. This mystery entails how God justifies sinners through faith alone in Christ, fulfilling His justice without compromising His holiness (Romans 3:26). It also includes the inclusion of Gentiles into God's saving plan (Ephesians 3:6), highlighting the universal scope of the gospel. Thus, understanding this mystery requires divine revelation, as human wisdom cannot grasp the depth of God's grace and mercy toward undeserving sinners (1 Corinthians 2:7-9).