A.T. Robertson Commentary Ephesians 6:20

"for which I am an ambassador in chains; that in it I may speak boldly, as I ought to speak." — Ephesians 6:20 (ASV)

For which I am an ambassador in chains (υπερ ου πρεσβευω εν αλυσε). "For which mystery" of the gospel (verse 19). Πρεσβευω is an old word for ambassador (from πρεσβυς, an old man) in N.T. only here and 2 Corinthians 5:20. Paul is now an old man (πρεσβυτης, Phm 1:9) and feels the dignity of his position as Christ's ambassador though "in a chain" (εν αλυσε, old word αλυσις, from α privative and λυω, to loosen). Paul will wear a chain at the close of his life in Rome (2 Timothy 1:16).

In it (εν αυτω). In the mystery of the gospel. This is probably a second purpose (ινα), the first for utterance (ινα δοθη), this for boldness (ινα παρρησιασωμα, first aorist middle subjunctive, old word to speak out boldly). See 1 Thessalonians 2:2. See Col 4:4 for "as I ought."