"MYSTERY"

The English word "mystery" is a transliteration of the Greek word musterion, which means a sacred secret.

Rather than write a new study on this topic, I will refer you instead to a complete explanation of it. Why repeat what has already been said, and that by a scholar with much greater qualifications and abilities than myself?
I will post more later on The Mystery and its meaning and application to our daily lives.
Read at this link:The Mystery
This is Appendix 193 From The Companion Bible by E. W. Bullinger
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I am greatly delighted that The Watchmen Group now houses the
complete list of appendices to this Study Bible.

The Companion Bible is currently in print, and is also available free online, although its sheer volume makes this latter seem rather impractical to use.
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E. W. Bullinger bio from Wikipedia.