The truth I say in Christ, etc. As it was an opinion entertained by most that Paul was, as it were, a sworn enemy to his own nation, and as it was suspected somewhat even by the household of faith, as though he had taught them to forsake Moses, he adopts a preface to prepare the minds of his readers, before he proceeds to his subject, and in this preface he frees himself from the false suspicion of evil will towards the Jews. And as the matter was not unworthy of an oath, and as he perceived that his affirmation would hardly be otherwise believed against a prejudice already entertained, he declares by an oath that he speaks the truth. By this example and the like, (as I reminded you in the first chapter,) we ought to learn that oaths are lawful, that is, when they render that truth credible which is necessary to be known, and which would not be otherwise believed. The expression, In Christ, means "according to Christ." [1] By adding I lie not, he signifies that he speaks without fiction or disguise. My conscience testifying to me, etc. By these words he calls his own conscience before the tribunal of God, for he brings in the Spirit as a witness to his feeling. He adduced the Spirit for this end, that he might more fully testify that he was free and pure from an evil disposition, and that he pleaded the cause of Christ under the guidance and direction of the Spirit of God. It often happens that a person, blinded by the passions of the flesh, (though not purposing to deceive,) knowingly and wilfully obscures the light of truth. But to swear by the name of God, in a proper sense of the word, is to call him as a witness for the purpose of confirming what is doubtful, and at the same time to bind ourselves over to his judgment, in case we say what is false.

Footnotes

1 - "Idem valet ac secundum Christum, -- it is the same with According to Christ;" "lego en Christo -- I speak in Christ," that is, as a Christian; to be in Christ and to be a Christian is the same. This idea bears on the import of the passage more than any other. It is as though he said, "Though I am in Christ or a Christian, yet I tell you this as the truth or the fact, and I have the testimony of conscience enlightened by the Spirit, that I have great grief and unceasing sorrow on your account." The Jews had the impression that the Apostle, having become the follower of Christ, must have necessarily entertained hatred towards them, and must have therefore felt no concern for them; for this is really the case with all real apostates, that is, with those who leave the truth for error, but not with them who leave error for the truth. To obviate this impression seems to have been the object here. How the idea of an oath comports with what follows it is difficult to see. It is no argument to say that what is here means the same as in Matthew 5:34, where it follows the verb "to swear." There is a passage similar to this in Ephesians 4:17; but en kurio there clearly signifies "by the Lord's authority." We may add, that to swear by Christ would have had no influence on the Jews. -- Ed.

I say the truth - In what I am about to affirm respecting my attachment to the nation and people.
In Christ - Most interpreters regard this as a form of an oath, as equivalent to calling Christ to witness. It is certainly to be regarded, in its obvious sense, as an appeal to Christ as the searcher of the heart, and as the judge of falsehood. Thus, the word translated "in" ἐν en is used in the form of an oath in Matthew 5:34-36; Revelation 10:6, Greek. We are to remember that the apostle was addressing those who had been Jews; and the expression has all the force of an oath "by the Messiah." This shows that it is right on great and solemn occasions, and in a solemn manner, and thus only, to appeal to Christ for the sincerity of our motives, and for the truth of what we say. And it shows further, that it is right to regard the Lord Jesus Christ as present with us, as searching the heart, as capable of detecting insincerity, hypocrisy, and perjury, and as therefore divine.
My conscience - Conscience is that act or judgment of the mind by which we decide on the lawfulness or unlawfulness of our actions, and by which we instantly approve or condemn them. It exists in every man, and is a strong witness to our integrity or to our guilt.
Bearing me witness - Testifying to the truth of what I say.
In the Holy Ghost - He does not say that he speaks the truth by or in the Holy Spirit, as he had said of Christ; but that the conscience pronounced its concurring testimony by the Holy Spirit; that is, conscience as enlightened and influenced by the Holy Spirit. It was not simply natural conscience, but it was conscience under the full influence of the Enlightener of the mind and Sanctifier of the heart. The reasons of this solemn asseveration are probably the following:
(1) His conduct and his doctrines had led some to believe that he was an apostate, and had lost his love for his countrymen. He had forsaken their institutions, and devoted himself to the salvation of the Gentiles. He here shows them that it was from no lack of love to them.
(2) the doctrines which he was about to state and defend were of a similar character; he was about to maintain that no small part of his own countrymen, notwithstanding their privileges, would be rejected and lost. In this solemn manner, therefore, he assures them that this doctrine had not been embraced because he did not love them, but because it was solemn, though most painful truth. He proceeds to enumerate their privileges as a people, and to show to them the strength and tenderness of his love.

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 (1) the truth in Christ, I lie not, my conscience also bearing me witness in the Holy Ghost,
(1) The third part of this epistle, which goes to the twelfth chapter, in which Paul ascends to the higher causes of faith: and first of all, because he purposed to speak much of the casting off of the Jews, he uses a declaration, saying by a double or triple oath, and by witnessing of his great desire towards their salvation, his singular love towards them, and in addition granting to them all their privileges.

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.

Being about to discuss the rejection of the Jews and the calling of the Gentiles, and to show that the whole agrees with the sovereign electing love of God, the apostle expresses strongly his affection for his people. He solemnly appeals to Christ; and his conscience, enlightened and directed by the Holy Spirit, bore witness to his sincerity. He would submit to be treated as "accursed," to be disgraced, crucified; and even for a time be in the deepest horror and distress; if he could rescue his nation from the destruction about to come upon them for their obstinate unbelief. To be insensible to the eternal condition of our fellow-creatures, is contrary both to the love required by the law, and the mercy of the gospel. They had long been professed worshippers of Jehovah. The law, and the national covenant which was grounded thereon, belonged to them. The temple worship was typical of salvation by the Messiah, and the means of communion with God. All the promises concerning Christ and his salvation were given to them. He is not only over all, as Mediator, but he is God blessed for ever.

THE BEARING OF THE FOREGOING TRUTHS UPON THE CONDITION AND DESTINY OF THE CHOSEN PEOPLE--ELECTION--THE CALLING OF THE GENTILES. (Romans. 9:1-33)
I say the truth in Christ--as if steeped in the spirit of Him who wept over impenitent and doomed Jerusalem (compare Romans 1:9; 2-Corinthians 12:19; Philippians 1:8).
my conscience bearing me witness in the Holy Ghost--"my conscience as quickened, illuminated, and even now under the direct operation of the Holy Ghost."

I say the truth in Christ. This affirmation is made so solemn because the Jews charged Paul with having forsaken his race. He speaks as in the presence of Christ, with a conscience enlightened by the Holy Spirit.
That I have great heaviness, etc. Not so much that his countrymen are estranged from him, as that they were without the blessing of Christ.
For I could wish myself accursed from Christ. He could wish this, if that would avail anything, to save his Jewish brethren.
Accursed. "Anathema," in the Revision. Rejected from Christ and lost.
My brethren. His Jewish brethren, those of the same Jewish stock as himself.
Who are Israelites. He now enumerates some of the glories of the Jewish race. Jacob, their ancestor, had been called Israel (Genesis 32:28) by the angel. This means a Prince with God, and this proud title was borne by his descendants.
Whose is the adoption. Six high privileges of the chosen people are named in Romans 9:4-5. They were adopted as the chosen people (Deuteronomy 7:6).
And the glory. The presence of the ark of God and the glory of the Divine Presence (1-Samuel 4:21).
The covenants. The covenants made with Abraham and at Sinai.
The giving of the law. The law of Moses given to the children of Israel.
And the service of God. The worship of the tabernacle and temple.
And the promises. Especially the blessed promise of Christ.
Whose are the fathers. The patriarchs and prophets.
Of whom . . . Christ came. Greatest of all, Christ, in his fleshly nature, was of their race, of the tribe of Judah, and of the seed of David.
Who is over all. See Matthew 28:18. He is our King and our Judge.
God blessed forever. More than man; Divine.

In Christ - This seems to imply an appeal to him. In the Holy Ghost - Through his grace.