God may open to us a door for the word: An open door symbolizes an opportunity to share the good news (see also Acts 14:27; 1 Corinthians 16:9; 2 Corinthians 2:12).
the mystery of Christ: See Colossians 1:26–27; 2:2.
for which I am in chains: Paul's commitment to spreading the good news led to his imprisonment (see Colossians Book Introduction, “Date and Occasion of Writing”).