A.T. Robertson Commentary Romans 9:1

"I say the truth in Christ, I lie not, my conscience bearing witness with me in the Holy Spirit," — Romans 9:1 (ASV)

In Christ (εν Χριστω). Paul really takes a triple oath here so strongly is he stirred. He makes a positive affirmation in Christ, a negative one (not lying), the appeal to his conscience as co-witness (συνμαρτυρουσης, genitive absolute as in 2:15 which see) "in the Holy Spirit."