Romans 9:1

I say the truth in Christ, I lie not, my conscience also bearing me witness in the Holy Ghost,

Paul solemnly assures the believers at Rome that what he is about to declare about his burden is true, calling Christ to witness while his conscience guided by the Holy Spirit confirms it. That affirmation introduces his profound and continual sorrow, a deep and unceasing grief of heart for his kinsmen according to the flesh. They are the Israelites who failed to recognize Jesus of Nazareth as the promised Messiah. Ever since his conversion he has longed for their salvation with more than ordinary desire to help his Jewish brethren gain a clear understanding of the gospel, constantly petitioning God on their behalf.

The verse shows conscience bearing witness under the Holy Spirit, teaching that the Spirit must control conscience. When so controlled, actions are approved by God-given reason and accord with truth and righteousness and mercy.