What does it mean that the gospel is a mystery revealed by God?

In Romans, Paul describes the gospel as a mystery that was kept secret since the world began but is now revealed to believers. This concept stresses that while the basic elements of the gospel can be understood by the intellect, the deeper truths about Christ and His work require the Holy Spirit's revelation to truly appreciate. The natural mind struggles with comprehending the fullness of the gospel due to its spiritual significance, which can only be perceived when God opens the eyes of the heart. This reveals that believers do not come to faith by mere human reasoning but through the sovereign act of God enlightening them to fully embrace the grace extended through Christ.