What is the mystery of the gospel mentioned in Romans 16?

The mystery of the gospel, as referenced in Romans 16:25-26, pertains to the hidden truths of salvation and God's plan which were previously kept secret. With the arrival of Christ and the proclamation of the gospel, these mysteries are made manifest to all nations. This revelation signifies that God's salvific work is no longer confined to Israel but is now accessible to Gentiles as well, indicating the inclusivity and universality of salvation in Christ. It's an important theological point that underscores the belief in predestination, where God has purposed to declare His grace to a chosen people across all ethnic and cultural backgrounds, prompting an obedience of faith.