'Mystery' in the Bible

Having made known unto us the mystery of his will, according to his good pleasure which he hath purposed in himself:

How that by revelation he made known unto me the mystery; (as I wrote afore in few words,

Whereby, when ye read, ye may understand my knowledge in the mystery of Christ)

which in other generations was not disclosed to mankind, as it has now been revealed to His holy apostles and prophets by the [Holy] Spirit;

And to make all men see what is the fellowship of the mystery, which from the beginning of the world hath been hid in God, who created all things by Jesus Christ:

This is a great mystery: but I speak concerning Christ and the church.

And for me, that utterance may be given unto me, that I may open my mouth boldly, to make known the mystery of the gospel,