I say the truth in Christ, I lie not - This is one of the most solemn oaths any man can possibly take. He appeals to Christ as the searcher of hearts that he tells the truth; asserts that his conscience was free from all guile in this matter, and that the Holy Ghost bore him testimony that what he said was true. Hence we find that the testimony of a man's own conscience, and the testimony of the Holy Ghost, are two distinct things, and that the apostle had both at the same time.
As the apostle had still remaining a very awful part of his commission to execute, namely, to declare to the Jews not only that God had chosen the Gentiles, but had rejected them because they had rejected Christ and his Gospel, it was necessary that he should assure them that however he had been persecuted by them because he had embraced the Gospel, yet it was so far from being a gratification to him that they had now fallen under the displeasure of God, that it was a subject of continual distress to his mind, and that it produced in him great heaviness and continual sorrow.

I say the truth in Christ, I lie not,.... The apostle being about to discourse concerning predestination, which he had mentioned in the preceding chapter, and to open the springs and causes of it, and also concerning the induration and rejection of the Jewish nation; he thought it necessary to preface his account of these things with some strong assurances of his great attachment to that people, and his affection for them, lest it should be thought he spoke out of prejudice to them; and well knowing in what situation he stood in with them, on account of his preaching up the abrogation of the ceremonial law, and how difficult it might be for him to obtain their belief in what he should say, he introduces it with a solemn oath, "I say the truth in Christ, I lie not": which refers not to what he had said in the foregoing chapter, but to what he was going to say; and is all one as if he had said, as I am in Christ, a converted person, one born again, and renewed in the spirit of my mind, what I am about to speak is truth, and no lie; or I swear by Christ the God of truth, who is truth itself, and I appeal to him as the true God, the searcher of hearts, that what I now deliver is truth, and nothing but truth, and has no falsehood in it. This both shows that the taking of an oath is lawful, and that Christ is truly God, by whom only persons ought to swear:

my conscience bearing me witness. The apostle, besides his appeal to Christ, calls his conscience to witness to the truth of his words; and this is as a thousand witnesses; there is in every man a conscience, which unless seared as with a red hot iron, will accuse or excuse, and bear a faithful testimony to words and actions; and especially a conscience enlightened, cleansed, and sanctified by the Spirit of God, as was the apostle's: hence he adds,

in the Holy Ghost; meaning either that his conscience was influenced and directed by the Holy Ghost in what he was about to say; or it bore witness in and with the Holy Ghost, and the Holy Ghost with that; so that here are three witnesses called in, Christ, conscience, and the Holy Ghost; and by three such witnesses, his words must be thought to be well established.