The word “mystery” in the NT refers to truth previously concealed but now presently revealed.
That definition is borne out in Matthew 13:11-17. Note especially verse 17–
For truly I say to you that many prophets and righteous men desired to see what you see, and did not see it, and to hear what you hear, and did not hear it.
Jesus here says that OT men of God had no knowledge of the truth that Jesus’ disciples now saw and heard. This is substantiated a little later by Matthew’s own assessment of Jesus’ teaching in parables–
This was to fulfill was what spoken through the prophet: “I will open My mouth in parables; I will utter things hidden since the foundation of the world (13:35).
Every NT use of the Greek word translated “mystery” has this sense–truth previously concealed but now presently revealed. Jesus spoke in parables so his followers would “see and hear” the truth while on the other hand hiding that same truth from his enemies (13:13).
Keeping that meaning and consistent use in mind would help in the interpretation of these parables Jesus spoke in Matthew 13. He was not detailing a mystery form of the kingdom but “the mysteries of the kingdom” (13:11).
The parables of the mysteries of the Kingdom reveal the state of the kingdom program during the “interregnum”–the period between the King’s ascension to heaven and later return to earth–gaining new citizens through the worldwide spread of the gospel (note especially 13:37-43).