1 I say the truth in Christ, I lie not, my conscience also bearing me witness in the Holy Ghost, (Romans 9:1 KJV)
William Burkitt’s Commentary
Observe here, 1. Apostle Paul’s solemn asseveration or oath; he calls Christ and the Holy Ghost to witness for the truth of what he says, which is the very formality of an oath.
Learn thence, That it is not barely lawful, but in some cases expedient and necessary to assert and confirm by oath the truth and certainty of what we speak; in cases of great moment, which cannot otherwise be sufficiently confirmed a Christian may establish his saying by an oath.
Observe, 2. The persons whom the apostle swears by, Christ and the Holy Ghost; he calls them to be the witness of the sincerity of his conscience in what he does assert: I say the truth in Christ, &c.
However, why does the apostle swear by the name of Christ, rather than by the name of God, since the Jews did not believe Jesus was divine, so they were not inclined to give more credit to what he attested?
I answer, Probably to assert the Godhead of Christ, which the Jews generally denied, and therefore wanted that honour which was and is due unto it. None but God was to be sworn by; the apostle swearing by Christ proves that Christ is truly God; as also the Holy Ghost; for an oath being an act of religious worship, and the apostle swearing by divinity, according to the words of Moses in Thou shalt fear the Lord thy God, and serve him, and shalt swear by his name. De 6:13
Observe, 3. As the apostle appeals to Christ and the Holy Ghost, so also to his own conscience, as the avoucher and witness of the truth of what he says, My conscience also bearing me witness.
Learn thence, That God has placed a conscience in every man, whose office it is to bear witness of all his words and actions; yea, of all his thoughts and inward affections. Conscience is God’s register, to record whatever we think, speak, or act; and happy he whose conscience bears witness for him, and does not testify against him; who can say with the apostle here, My conscience beareth me witness, that I lie not, but say the truth in Christ.