praying: Romans 15:30-32; Ephesians 6:19; Philippians 1:19; 1 Thessalonians 5:25; Philemon 1:22; Hebrews 13:18-19
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: Colossians 1:26, Colossians 2:2-3; Matthew 13:11; 1 Corinthians 4:1; Ephesians 6:19