N Ephesians 3 Paul refers to the-gospel-to-the-world as a “mystery” – a hidden secret (vs. 3-6 & 9). While every accepted saint in every age has been given God's grace, & while every child of God in every dispensation is justified by faith, & while the death of Christ is the sufficient agent of change in the condition of every devotee (vs. 15), the full understanding (of how God accomplishes redemption & applies His love) has not been fully understood by all (vs. 5). But Jesus revealed this mystery to Paul with full clarity (vs. 3-4). Amazingly, we (through Paul's writings) likewise have this access. We get to see the full picture of God's plan of redemption. So, we fellowship joyfully with God & with one another around the glorious voluntary sacrifice of His Son (vs. 9).
In our era (the age of the church), God's amazing love has been discovered primarily by non-Jews (vs. 6). Paul calls this revealed mystery of the gospel the "unsearchable riches of Christ" (vs. 8). Yet, with God all things are possible. So, through revelation, that which is unsearchable can be seen clearly (vs. 9). Or, to repeat Paul’s most excellent words: "Know the love of Christ, which passes knowledge, that you might be filled with all the fullness of God" (vs. 19). How can we know the unknowable? God reveals His love to us – as undeserving as we are – and so experientially we get to taste the most spectacular offering in (or out) of this world.