# For this purpose

"For this" or "For this reason"

# I was appointed a herald and an apostle

"Christ appointed me to be a herald and an apostle" or "Christ made me a preacher and an apostle" 

# a herald

"an official messenger." 

# I am telling the truth, I am not lying

With these two comments Paul emphasizes that he is telling the truth when he says that he was appointed to be a herald, an apostle, and a teacher. Some translators may prefer to put these comments at the end of the verse.

# in faith and truth

"about the message of faith and truth" or "about the true faith"