Why did Paul say, in Galatians 1:20, “I assure you before God that what I am writing you is no lie.” He would never lie for he was sent forth by the Lord.
We will need to quote the entire context in which this verse is found to answer your question. Before we do it is important to know that after Paul’s initial visit to them the churches of Galatia were visited by “false Jewish teachers” who introduced a “false gospel” to the saints (see 1:6-10; 3:1; 4:8-11; 5:1-4, 7-10; 6:11-12). They knew what Paul had taught them (which was the “gospel of Christ”) and so they tried to turn them against Paul and his message by undermining his apostleship (4:16-17). Now we will read 1:11-20 and we will see that Paul defends his apostleship with passion and assures them that he didn’t receive his calling from man, but directly from the Lord.
“But I make known to you, brethren, that the gospel which was preached by me is NOT ACCORDING TO MAN. For I NEITHER RECEIVED IT FROM MAN, NOR WAS I TAUGHT IT, but IT CAME THROUGH THE REVELATION OF JESUS CHRIST. For you have heard of my former conduct in Judaism, how I persecuted the church of God beyond measure and tried to destroy it. And I advanced in Judaism beyond many of my contemporaries in my own nation, being more exceedingly zealous for the traditions of my fathers. But when it pleased God, who separated me from my mother’s womb and CALLED ME BY HIS GRACE, to reveal His Son in me, that I might preach Him among the Gentiles, I did not immediately confer with flesh and blood, nor did I go up to Jerusalem to those who were apostles before me; but I went to Arabia, and returned again to Damascus. Then after three years I went up to Jerusalem to see Peter, and remained with him fifteen days. But I saw none of the other apostles except James, the Lord’s brother. (Now concerning the things which I write to you, indeed, before God, I do not lie).”
Paul’s defense of his call as an apostle is strong and one that contradicted the Jewish teachers who tried to discredit Paul and his message. The Jewish teachers no doubt told the Galatians that Paul could not be an apostle since he had never even visited the apostles to get their APPROVAL. Paul shows them in these verses that he didn’t need man’s approval or consent to preach and teach; he received it as a direct revelation from the ascended Lord! (We read of his conversion and his calling as an apostle to the Gentiles in Acts 9:1-15.) Three years later he did visit the apostle Peter, but it was NOT an “official visit” to receive permission to preach; it was a “private visit” to meet him and to get to know him. We read that he “went up TO SEE Peter” and W. E. Vine tells us (in his Expository Dictionary) that the words “to see” is the Greek word HISTOREO which means, “to visit in order to become acquainted with.” Because of the opposition he was receiving from false teachers and the apparent opposition from some of the Galatians who were being influenced by them, Paul said, “the things which I write to you, indeed, before God, I do not lie.” His message was strong, and he punctuated it with a strong affirmation that he was telling the truth! Did they believe Paul when they read his letter? We aren’t sure, but they should have. He was “giving them HIS WORD” that his words were the words of truth. God was his WITNESS that he was telling the truth and that should have settled the matter and silenced all who opposed him and his message. They should have acknowledged that Paul’s apostleship and his gospel message was “independent of man”; it was “all of God.” (DO) (505.1)