This can be stated in active form. Alternate translation: "Christ appointed me to be a herald and an apostle" or "Christ made me a preacher and an apostle" (See: [[rc://en/ta/man/translate/figs-activepassive]])
"an official messenger." A herald is an official messenger who is sent to bring news.
# 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
This probably refers to what Paul was to teach about. He may be using "faith" and truth" to express one idea. Alternate translation: "about the message of faith and truth" or "about the true faith" (See: [[rc://en/ta/man/translate/figs-hendiadys]])