The Apostle Paul speaking in Colossians 1:25-26 says,
"Of the church I was made a minister according to the stewardship from God
bestowed on me for your benefit, that I might fully carry out the preaching
of the word of God, that is, the mystery which has been hidden from ages and
generations; but has now been manifested to His saints."

What is the mystery? It is the hidden, eternal plan of God that is being
revealed to God's people. It is according to His time and in His way. It is
an open secret revealed to His servants through His Holy Spirit. Let's take
a brief look to see what Scripture opens up to us. As always, I urge you to
search these things out for yourself. You will be blessed!

To the disciples Jesus said,
"...To you it has been granted to know the mysteries of the kingdom of
 heaven..." Mathew 13:11
"...To you has been given the mystery f the kingdom of God..." Mark 4:11
"...To you it has been granted to know the mysteries of the Kingdom of 
God..."  Luke 8:10
Scripture tells us, to all others Jesus spoke in parables.

What then is this mystery? "Christ in you the hope of glory."
"To them (His saints, holy ones, true believers) God willed to make known
what is the riches of the glory of this mystery among the Gentiles: which is
Christ in you the hope of glory." Colossians 1:27 (My parentheses)

Concerning the indwelling of Christ--
We see it is revealed in the coming of the Holy Spirit as Jesus tells us in
John 14:16-17:
"And I will ask the Father, and He will give you another Helper...the Spirit
of Truth...He abides with you, and will be in you."
Jesus goes on to say,
"In that day you shall know that I am in My Father and you in Me, and I in
you." John 14:20

We see it demonstrates to the world Christ's mission:
"I in them, and Thou in Me, that they may be perfected in unity; that the
world may know that Thou didst send Me, and didst loved them, even as Thou
didst love Me. John 17:23
(Can you see here why we are admonished so often to love one another? It is
the unity, oneness, of Believers that reveals God's love for all.)

We see the old life dies that the new may live:
"I have been crucified with Christ; and it is no longer I who live, but
Christ lives in me; and the life which I now live in the flesh I live by
faith of the Son of God, who loved me, and delivered Himself up for me."
Galatians 2:20

We see it is obtained by faith:
"So that Christ may dwell in your hearts through faith; and that you, being
rooted and grounded in love may be able to comprehend with all the saints
what is the breadth and length and height and depth, and to know the love of
Christ which surpasses knowledge that you may be filled up to all the
fullness of God." Ephesians 3:17-19

We see obedience is essential:
"And the one who keeps His commandments abides in Him, and He in him. And 
we know by this He abides in us, by the Spirit whom He has given us."
1 John 3:24

We see it is possible for all:
"Behold I stand at the door and knock; if anyone hears My voice and opens
the door, I will come in to him, and will dine with him, and he with Me."
Revelation 3:20

Scripture reveals that all things are summed up in Christ Jesus our Lord!
"He made known to us the mystery of His will, according to His kind
intention which He purposed in Himself with a view to an administration
suitable to the fullness of the times, that is, the summing up of all things
in Christ, things in the heavens and things upon the earth." Ephesians 1:910

"For in Him all the fullness of Deity dwells in bodily form, and in Him you
have been made complete, and He is the head over all rule and authority."
Colossians 2:9-10

To know we are a part of God's glorious plan from eternity is awesome beyond
measure! With all humility we thank our loving and gracious Father for His
plan, our precious Lord Jesus for providing the way through His work on the
Cross and our faithful Holy Spirit for revealing truth to us.

This day, wherever you may be, let's join our voices in praise and worship
to Almighty God. For it is His indwelling love that fills us with such a
desire to just that! Amen