This was according to the eternal purpose that he has realized in Christ Jesus our Lord, in whom we have boldness and access with confidence through our faith in him. So I ask you not to lose heart over what I am suffering for you, which is your glory.Ephesians 3:11-13
It is in Christ Jesus our Lord that "we have boldness and access with confidence through our faith in him". We were once dead in our trespasses and sins and children of wrath. We had no right standing before God. However, we were made alive together with Christ. We were raised up with him and seated with him. We were reconciled to God and it is through him that we have access to the father. Jesus became our righteousness and on the basis of his righteousness we have a right standing before God. We can come before him without shame or fear. As Paul says here, we can have boldness and access with confidence. There is nothing to fear because of Jesus.
Paul has been telling them about how he's a prisoner and about the mystery that was made known. Paul was made a minister of the gospel, given the privilege of serving God by spreading the Good News. Paul sees some of the fruit and purposes of this duty. This job he has been doing is to bring to light for everyone this mystery that was hidden. He is preaching the unsearchable riches of Christ. There is a greater purpose of the manifold wisdom of God being made known to the angelic rulers and authorities. From all this, I understand Paul to be saying that this is a great joy to him and it has a purpose beyond just his suffering. This is probably why he encourages them "not to lose heart over what I am suffering for you". He tells them that this suffering "is your glory". They should feel honored because of it. I assume that he is in prison for spreading the gospel. I guess in that sense it is because of them and/or for them.