A mystery is a secret that has been told
Paul's mysteries are Gentiles brought into the same body; Christ in you; the resurrection of the living. Each time he names a mystery he immediately declares it. The word marks something that could not have been worked out and had to be revealed.
“that is, the mystery made known to me by revelation, as I have already written briefly.”
Ephesians 3:3→
“the mystery that was hidden for ages and generations but is now revealed to His saints.”
Colossians 1:26→
“Listen, I tell you a mystery: We will not all sleep, but we will all be changed—”
1 Corinthians 15:51→
