What is meant by the gospel being called a mystery?

In the sermon, the gospel is described as a 'mystery' because its truths are not apprehended by natural understanding or education but require divine revelation. This mystery includes God's plans for salvation through Christ, which was hidden from many and yet made known to His elect. As Paul states, the wisdom of God often appears foolish to the world, demonstrating that the knowledge of salvation comes through the Spirit and the Word (1 Corinthians 1:18). Therefore, believers celebrate the mystery of godliness as they come to know Christ more deeply.