Paul is writing to Gentile converts, and the specific mystery that he is talking about here—our hope—is Christ in us. It was no secret in the Old Testament that the Gentiles would be saved, but that Christ, by means of the Holy Spirit, would dwell in all converts was unknown. Otherwise, Paul would be telling a lie when he said that it is a mystery now revealed to the church. This applies directly to us because it is Christ living His life in us that prepares us for the Kingdom of God. God in us is our hope!
This hope gives us certainty for the future, which carries beyond the grave, even as it did with Christ. He rose from the grave because God was in Him. This hope is why we can have joyful and confident expectation of salvation because Christ's life—His character, values, virtues, thoughts, attitudes, and deeds—can become evident in a Christian.
John W. Ritenbaugh
Perseverance and Hope