2 Corinthians 5:20 (a) This title is given to those Christians who carry GOD's message to a lost and hostile world. It probably does not apply to all believers for many of GOD's children are afraid to become His messengers, and they keep the good news to themselves The true ambassador comes out boldly for his king and for his country.
Ephesians 6:20 (a) Paul used the title in this passage because he was representing Heaven on earth. He carried the King's message to the rebels who were bent on killing him. He was GOD's representative to bring to men the Word of his Lord both for their salvation and their condemnation.