Paul was an apostle
by the will of God!
An apostle is one
sent to speak the
words of someone else! The word is also used for
Jesus who was sent from God. (Hebrews 3:1) Paul is writing from prison.
Why would the Holy Spirit guide Paul to use the
word Christ in this letter?
Paul only uses Jesus without the word Christ in one verse in the book
of Ephesians.
You will have to find the verse.
The word Christ
means anointed one
(a priest or king). Jesus is both priest and king.