'Mystery' in the Bible

making known to us the mystery of his will, according to his good pleasure that he purposed in him,

According to revelation the mystery was made known to me, just as I wrote beforehand in brief,

so that you may be able when you read to understand my insight into the mystery of Christ

and to enlighten everyone [as to] what [is] the administration of the mystery hidden from the ages by God, who created all [things],

(This mystery is great, but I am speaking with reference to Christ and the church.)

and for me, that a word may be given to me at the opening of my mouth, to make known with boldness the mystery of the gospel,