Now the things which I write to you. This affirmation extends to the whole narrative. The vast earnestness of Paul on this subject is evinced by his resorting to an oath, which cannot lawfully be employed but on great and weighty occasions. Nor is it wonderful that he insists with so much earnestness on this point; for we have already seen to what expedients the impostors had recourse in order to take from him the name and credit of an apostle. Now the modes of swearing used by good men deserve our attention; for we learn from them that an oath must be viewed simply as an appeal to the judgment-seat of God for the integrity and truth of our words and actions; and such a transaction ought to be guided by religion and the fear of God.

Behold, before God I lie not - This is an oath, or a solemn appeal to God; see the note at Romans 9:1. The design of this oath here is to prevent all suspicion of falsehood, It may seem to be remarkable that Paul should make this solemn appeal to God in this argument, and in the narrative of a plain fact, when his statement could hardly be called in question by anyone. But we may remark:
(1) That the oath here refers not only to the fact that he was with Peter and James only fifteen days, but to the entire group of facts to which he had referred in this chapter. "The things which I wrote unto you." It included, therefore, the narrative about his conversion, and the direct revelation which he had from the Lord Jesus.
(2) there were no radios which he could appeal to in this case, and he could, therefore, only appeal to God. It was probably not practicable for him to appeal to Peter or James, since neither of them were in Galatia, and a considerable part of the transactions here referred to occurred where there were no witnesses. It pertained to the direct revelation of truth from the Lord Jesus. The only way, therefore, was for Paul to appeal directly to God for the truth of what he said.
(3) the importance of the truth here affirmed was such as to justify this solemn appeal to God. It was an extraordinary and miraculous revelation of the truth by Jesus Christ himself. He received information of the truth of Christianity from no human being. He had consulted no one in regard to its nature. That fact was so extraordinary, and it was so remarkable that the system thus communicated to him should harmonize so entirely with that taught by the other apostles with whom he had had no contact, that it was not improper to appeal to God in this solemn manner. It was, therefore, no trifling matter in which Paul appealed to God; and a solemn appeal of the same nature and in the same circumstances can never be improper.

Before God I lie not - This he speaks in reference to having seen only Peter and James at Jerusalem; and consequently to prove that he had not learned the Gospel from the assembly of the apostles at Jerusalem, nor consequently received his commission from them.

Now the things which I write unto you, behold, (o) before God, I lie not.
(o) This is a type of an oath.

Now the things which I write unto you,.... Concerning his education, his religion, his principles and practices before conversion; concerning his call by the grace of God, the revelation of Christ in him, and his preaching of him among the Heathen; concerning his travels to several places for this purpose, and especially concerning his not receiving the Gospel from men, not from any of the apostles; and how that upon his conversion he did not go up to Jerusalem to any of them, to be taught and sent forth by them; and that it was not till three years after that he wept thither to see Peter, with whom he stayed but fifteen days, and saw no other apostle, but James the Lord's brother. Now this being a matter of moment, and what he had been charged with by the false teachers, that the Gospel he preached he had received from men, in order to disqualify him and bring him into contempt as an apostle, and which they had insinuated to the Galatians; he therefore not only wrote these things, but for the confirmation of them solemnly appeals to God the searcher of hearts for the truth of them;
behold, before God I lie not; which is not only a strong asseveration, but a formal oath; it is swearing by the God of truth, calling him to be witness of the things that he had written; whence it is manifest that an oath upon proper occasions, where there is a necessity for it, and a good end to be answered by it, may be lawfully made.

Solemn asseveration that his statement is true that his visit was but for fifteen days and that he saw no apostle save Peter and James. Probably it had been reported by Judaizers that he had received a long course of instruction from the apostles in Jerusalem from the first; hence his earnestness in asserting the contrary facts.