The word “mystery” describes a hidden or secret thing, known only to the initiated. In Scripture it stands in contrast to the manifest or public dealings of God. The Lord Jesus, having been rejected, is now hidden in the heavens, and the ways of God are secret to the world but made known — as also His hidden purpose which is being accomplished by His secret ways — to those who have “ears to hear.”
The Lord often spoke in parables to the multitude, but explained them to the apostles, because it was given to them to know the mysteries of the kingdom. To them He said, “Unto you it is given to know the mystery of the kingdom of God: but unto them that are without, all these things are done in parables.” Christianity is a mystery to the unconverted. The apostles were stewards of the mysteries of God, and they spoke “the wisdom of God in a mystery.” The Apostle Paul spoke of the “mystery of the gospel,” the “mystery of the faith,” the “mystery of Christ,” and the “mystery of godliness.”
The marvelous purpose of God, the mystery of the church that had been hidden for ages, was revealed to Paul, as well as its present administration. The intelligence of it explains how Christ can be here in a scene from which He has been rejected alike by Jew and Gentile. Though there are several things designated mysteries, yet God in His grace has made them known to His saints.
Adapted from Concise Bible Dictionary