“the secret that hath been hid from the ages and from the generations, but now was manifested to His saints,” (Colossians 1:26)

“that by revelation He made known to me the secret, according as I wrote before in few [words] — in regard to which ye are able, reading [it], to understand my knowledge in the secret of the Christ, which in other generations was not made known to the sons of men, as it was now revealed to His holy apostles and prophets in the Spirit” (Ephesians 3:3 – 5)

It is also called the “Purpose of the ages” (literally) found in the verses below (original Greek & literal translations only):

“and to cause all to see what [is] the fellowship of the secret that hath been hid from the ages in God, Who the all things did create by Jesus Christ, that there might be made known now to the principalities and the authorities in the heavenly [places], through the assembly, the manifold wisdom of God, according to the Purpose of the ages, which He made in Christ Jesus our Lord,” (Ephesians 3:9 – 11)

The “Secret” or “Mystery” of Christ centers around the “One Purpose” or “Secret Will” of His to gather in “One All Things” in Christ in the “fullness” of the “times” (ages to come – Ephesians 2:7)

“having made known to us the Secret of His will, according to His good pleasure, that He purposed in Himself, in regard to the dispensation of the fullness of the times, to bring into one the all things in the Christ, both the things in the heavens, and the things upon the earth — in Him;” (Ephesians 1:9 – 10)

Where by His Spirit we can see that the “gathering of all things in One in Christ” is to “Reconcile them All”:

“and through Him to Reconcile the all things to Himself — having made Peace through the Blood of His Cross — through Him, whether the things upon the earth, whether the things in the heavens.” (Colossians 1:20)

And finally in the “end” (of the ages when His Purposes are accomplished), God becomes “All in All”. Indeed, here’s the “ending” verse:

“and when the all things may be subjected to Him, then the Son also Himself shall be subject to Him, Who did Subject to Him the all things, that God may be the all in all.” (1 Corinthians 15:28)