What has been secretly kept in God's mind until the right time?
The mystery—the church and all events connected with the church—was the secret kept in God's mind. Paul uses “mystery” for this hidden truth unique to his writings. It was not revealed until God saw fit, remaining concealed until His appointed time.
Les explains that in Romans 11:25 Paul speaks of a mystery, a word unique to Paul's writings, and it refers to the secret that has been kept in the mind of God. Romans 11 deals with Israel's future and how she will be grafted in again. Les stresses that the church and all the events connected with the church are secret. They have been kept in the mind of God, not to be revealed until God saw fit.
For I would not, brethren, that ye should be ignorant of this mystery, lest ye should be wise in your own conceits; that blindness in part is happened to Israel, until the fulness of the Gentiles be come in.