Romans 9:1 I speak the truth in Christ - I am not lying, my conscience confirms it through the Holy Spirit -
What do you say when you really want someone to believe what you are about to tell them is the truth? Do you swear on a parent’s life or other strange thing?
The apostle wanted the people to believe in the birth, death and resurrection of Christ and backed it up with a confirmation the Holy Spirit was guiding him.
He was not just saying what someone else wanted him to say; he believed what he was saying was the truth and his conscience knew it to be true.