Mystery
The 'mystery' refers to the previously hidden truth that Gentiles are fellow heirs, members of the same body, and partakers of the promise in Christ Jesus through the gospel.
This symbol underscores the inclusivity of the gospel and the unity of Jews and Gentiles in Christ, reflecting God's eternal plan of salvation for all humanity.
In the first-century context, the division between Jews and Gentiles was significant, making the revelation of their unity in Christ a profound and radical truth.