a door of utterance - Translate, “a door for the word.” Not as in Eph 6:19, where power of “utterance” is his petition. Here it is an opportunity for preaching the word, which would be best afforded by his release from prison (1Co 16:9; 2Co 2:12; Phm 1:22; Rev 3:8).
to speak - so that we may speak.
the mystery of Christ - (Col 1:27).
for which ... also - on account of which I am (not only “an ambassador,” Eph 6:20, but) ALSO in bonds.