The Church and its message was a mystery
in the Old Testament.

Paul says in Ephesians 3:1-6
"For this cause I Paul, the prisoner of Jesus
Christ for you Gentiles,
If ye have heard of the dispensation of the
grace of God which is given me to you-ward:
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 ages was not made known
unto the sons of men, as it is now revealed
unto his holy apostles and prophets by the
Spirit;
That the Gentiles should be fellowheirs, and
of the same body, and partakers of his
promise in Christ by the gospel:"