Below is from the DubiousDesciple.com
Romans 9:1, Paul the Liar?
I speak the truth in Christ—I am not lying, my conscience confirms it through the Holy Spirit. – Romans 9:1
Have you ever noticed how often Paul insists he is not lying? Who exactly was calling him a liar? Who did he feel the need to defend against in his letters?
Speculation abounds. Some believe Paul’s “thorn in the flesh” (2 Cor 12:7) to be whatever person or group Paul constantly defends himself against. Paul’s letters betray the tension between himself and Judaic Christians, and one Christian branch–the Ebionites–were not shy about their feelings toward Paul. They distrusted him. But we may never know exactly who he was referring to in verses like today’s.
Here are a few more verses where Paul insists he’s telling the truth:
By honor and dishonor, by evil report and good report; as deceivers, and yet true; – 2 Corinthians 6:8
The God and Father of our Lord Jesus Christ, who is blessed forever, knows that I am not lying. – 2 Corinthians 11:31
Now concerning the things which I write to you, indeed, before God, I do not lie. – Galatians 1:20
1 Timothy 2:7
For this reason I was appointed as a preacher, an apostle, and a faithful and true teacher of the Gentiles. I am telling the truth; I am not lying about anything.