1, 2. (a) How has the word “mystery” been defined? (b) What purpose in connection with the Christ has God made an open secret to us?

A MYSTERY has been defined as “any truth unknowable except by revelation of God.” It is a “sacred secret” that is revealed by God in his own due time. (Romans 16:25, 26) For long periods of time it was a mystery or sacred secret just who would be the Messiah, the “seed” of God’s heavenly “woman.” Also, God’s purpose in connection with the Messiah or Christ was a long-time mystery or sacred secret. But in his appointed time God revealed, or kept no longer a secret, that it was his purpose to use the Messiah or Christ in connection with an administration of all things, as in the management of a house by a steward. Such an administration for unity would mean that God would head up all things in the Messiah (Christ) or gather all things together again under the headship of the Messiah or Christ. It was a kindness for God as the Administrator to reveal this, just as we read:

for an administration [management as by a steward] at the full limit of the appointed times, namely, to gather all things together again in the Christ [Ma·shiʹahh], the things in the heavens and the things on the earth. Yes, in him,

Ephesians 1:8-12.