The phrase "justified in Christ" means justified because we are united with Christ and justified by means of Christ.
# we too, were found to be sinners
The words "were found to be" are an idiom that emphasizes that "we are" certainly sinners. AT: "we see that we also certainly are sinners" (See: [[rc://en/ta/man/translate/figs-idiom]])
# Absolutely not!
"Of course, that is not true!" This expression gives the strongest possible negative answer to the preceding rhetorical question "Does Christ become a servant of sin?" You may have a similar expression in your language that you could use here. (See: [[rc://en/ta/man/translate/figs-rquestion]])