Under the terms of the Apostle Paul's "gospel of the grace of God" (Acts 20:24) in this day of "the dispensation of the grace of God" (Eph 3:2), it is only by the pure grace of God "in Christ" that believers may stand "holy & blameless" (Eph 1:4) before Him. Believers then "become rooted and grounded 'in Christ," growing in the knowledge and grace of God. We "grow up into him (Christ) in all things..." (Eph 4:15).

For this to occur, every believer needs to first receive Christ, and then to have  "the REVELATION of the 'mystery," That mystery is simply God's long held secret eternal plan.

Romans 16:25 Now to him that is of power to stablish you according to my (Paul's) gospel, and the preaching of Jesus Christ, according to the REVELATION OF THE MYSTERY (Gk., musterion, a God ordained secret), which was Kept Secret Since the World Began,

The "Relation of the Mystery" is that Christ's own resurrection life may now be deposited into believers the moment they believe (cf. Col 1:25-27). Christ then becomes the believer's indwelling life-supply; enabling them, by their union with Him, live righteous lives and enjoy His rest and His peace.

I often use the term 'pure grace' to emphasize that grace, the gift of Christ's life must be received as free and unadulterated. 'Pure grace' can't involve any amount of works or law-keeping or it won't be grace at all. Paul clearly says religious 'law & works' are in opposition to the 'pure grace' of God.

Romans 6:14b … for ye (believers) are NOT UNDER THE LAW, but UNDER GRACE.

Romans 11:6 And IF BY GRACE, then is it NO MORE OF WORKS: otherwise grace is no more grace. But if it be of works, then is it no more grace: otherwise work is no more work.

The most significant measure of God's grace, which is seen as God's "manifold wisdom" and "eternal purpose," is what Paul calls "the Fellowship of the 'Mystery' (the secreted plan of God)." This truth was first revealed to Paul for us as grace believers today. In fact Paul says his ministry was "To Make All Men See what is the Fellowship Of The Mystery," which he tells us is "Christ In You" (Col 1:27). This was God's eternal and ultimate plan that was "purposed in Christ Jesus" (Eph 3:11, below) "before the world began" (Rom 16:25, above)

Ephes. 3:9-11 And to MAKE ALL MEN SEE what is the FELLOWSHIP OF THE MYSTERY, which from the beginning of the world hath been hid in God, who created all things by Jesus Christ: 10To the intent (hina, to demonstrate) that (that fellowship) now unto the principalities and powers in heavenly places might be known by the church (by the body of Christ) the manifold (multiple aspects) of wisdom of God, 11According to the Eternal Purpose which he Purposed in Christ Jesus… 

"Fellowship" - Louw & Nida Greek Lexicon of the Bible: an association involving CLOSE MUTUAL RELATIONS and involvement (taking part) - close association. 1John 1:3; … and truly our fellowship (close mutual relations and involvement, taking part) is with the Father, and with his Son Jesus Christ.

The "fellowship of the mystery" is the continual, unbreakable, close mutual relationship between Christ and every believer in whom His resurrected "Spirit of life" dwells. Believers may foolishly choose to depart from that union at times... but He will never depart from our spirit in which He eternally dwells.

Thus every believer may enjoy this union and the assurance, guidance and rich supply of the Lord... but they must come to 'know' the truth of His indwelling, which the Apostle Paul called "the Mystery" (Gk., musterion, God's secret plan).

Paul clearly defines the subjective personal aspect of "the Mystery, which is CHRIST IN YOU" (Col 1:27) in his thirteen epistles for the members of "the church, which is body of Christ" (Eph 1:22b-23a). This matter of the indwelling is the literal deposit of the "Spirit of life in Christ Jesus" (Rom 8:2a) into the believer's human spirit by which believers become "one spirit" (1Cor 6:17) with Christ's Spirit.

Intimacy may sound like its a little too close for some, but consider that each child of God is literally born of the Spirit-loins of God the Father by the deposit of His own "seed, which is Christ" (Gal 3:16, 1Pet 1:23).