a door: The term door is used metaphorically for an entrance to any business, or occasion or opportunity of doing any thing; and consequently "a door of utterance" is an opportunity of preaching the gospel successfully. See the parallel texts.
the mystery: Col 1:26, Col 2:2, Col 2:3, Mat 13:11, 1Co 4:1, Eph 6:19
Reciprocal: Isa 57:19 - the fruit Eze 29:21 - the opening Mat 6:5 - when Luk 21:15 - I will Joh 10:3 - the porter Act 10:27 - and found Act 14:27 - opened Act 28:31 - with 1Co 1:5 - in all 2Co 1:11 - helping Eph 3:4 - the mystery