en_tn_condensed/col/04/03.md

God would open a door

Opening a door for someone is a metaphor for giving that person the opportunity to do something. Alternate translation: "God would provide opportunities" (See: rc://en/ta/man/translate/figs-metaphor)

open a door for the word

"make an opportunity for us tp preach his message"

the secret truth of Christ

This refers to the gospel of Jesus Christ, which was not understood before Christ came.

Because of this, I am chained up

Here "chained" is a metonym for being in prison. Alternate translation: "It is for proclaiming the message of Jesus Christ that I am now in prison" (See: rc://en/ta/man/translate/figs-metonymy)