The gospel is a glorious mystery. That is the testimony of today’s text. This does not mean that it is beyond all understanding, for the apostle says that God has made known to us the riches of this mystery. In Scripture, a mystery is a truth that is not discoverable by human reason but that has been made known by divine revelation. In this sense, the gospel is a mystery, for its truth comes to us by pure revelation. And, as I have said, it is a rich and glorious revelation.
How glorious is this! “Christ in you.” We must not divorce these words from all that goes before in the chapter. Paul has spoken much of Christ: His eternal deity; His absolute sovereignty over all creation, for He is its Creator and upholder; His headship over the church that He purchased with His own blood and for which He made a perfect atonement. Now, after all this, Paul says, “Christ in you.” That is, Christ dwells in you by His Spirit and grace and holds you in perfect union with Himself. This is why Paul could tell the Corinthians that believers are “the temple of God, and the Spirit of God dwelleth in you” (1 Corinthians 3:16).
“Christ in you [is] the hope of glory.” That is, all in whom Christ dwells will certainly dwell with Him in glory. Grace is glory in the bud. All who have Christ have eternal life and are sure of glory (1 John 5:12).
What manner of men then ought believers to be? We should live with the conscious realisation that “Christ is in me.” We should live so as not to defile the temple of God. Since by indwelling us Christ guarantees us glory, we should walk on earth as the citizens of heaven.