What does it mean that the church is a mystery?
The church as a mystery means it is a secret kept in the mind of God, not revealed until God saw fit. The term is unique to Paul's writings, and the church and events connected with it remained hidden in God's purpose.
Les explains that 'mystery' is a word unique to Paul's writings and refers to the secret that has been kept in the mind of God. Les stresses that the church and all events connected with the church are secret; they were kept in the mind of God and not to be revealed until God saw fit. In the Romans 11 context, Paul deals with Israel's future and how she will be grafted in again, and then in verse 25 he introduces this mystery.
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.