From the beginning, Paul had purely and faithfully instructed the churches of Galatia in the Gospel, which they received with great enthusiasm (4:14-15).
However, soon after his departure false teachers (i.e., Judaizers; Jewish Christians zealous to maintain Jewish customs and traditions) infiltrated the churches and introduced a false gospel in which they added works along with faith as the ground of justification before God (1:7; 4:10; 5:10, 12).
By submitting to this false gospel, Paul warned the Galatians they would in fact be turning away from God (1:6), trading their freedom for bondage (4:9; 5:1), severing themselves from Christ (5:2, 4) and incurring the curse of God’s law (1:6-9; 3:10). This false Gospel stood completely contrary to the Gospel of grace, which Paul had originally proclaimed to them (2:16; 5:2-4).
There are two important issues that Paul deals with in his letter.
First, he concerns himself with a personal issue, the authenticity of his apostleship. Second, Paul deals with a theological issue, the authenticity of his Gospel, specifically justification through faith in Christ alone apart from works of the law (2:16).
In order to establish their false gospel and to gain credibility among the Galatians, it was necessary for the Judaizers to discredit Paul. So they sought to diminish his status among his Galatian converts and to call into question his authenticity as a true Apostle of Christ.
F.F. Bruce writes,
These Judaizers argued: ‘The Jerusalem leaders are the only persons with authority to say what the true gospel is, and this authority they received direct from Christ. Paul has no comparable authority: any commission he exercises was derived by him from the Jerusalem leaders, and if he differs from them on the content or implications of the gospel, he is acting and teaching quite arbitrarily. In fact,’ they may have added, ‘Paul went up to Jerusalem shortly after his conversion and spent some time with the apostles there. They instructed him in the first principles of the gospel and, seeing that he was a man of uncommon intellect, magnanimously wiped out from their minds his record as a persecutor and authorized him to preach to others the gospel which he had learned from them. But when he left Jerusalem for Syria and Cilicia he began to adapt the gospel to make it palatable to Gentiles. The Jerusalem leaders practiced circumcision and observed the law and the customs, but Paul struck out on a line of his own, omitting circumcision and other ancient observances from the message he preached, and thus he betrayed his ancestral heritage. This law-free gospel has no authority but his own; he certainly did not receive it from the apostles, who disapproved of his course of action. Their disapproval was publicly shown on one occasion at Antioch, when there was a direct confrontation between Peter and him on the necessity of maintaining the Jewish food-laws,’ (Commentary on Galatians, NIGTC, p. 26).
In response to these personal attacks, Paul includes a significant autobiographical section in order to vindicate his apostolic authority by establishing his independence from the leaders of the Jerusalem church, chiefly James, Cephas and John (1:10-2:21).
However, to truly understand the letter of Galatians, one must understand that this autobiographical section is more than Paul’s self-defense against the personal attacks of the false teachers.
Paul includes this extended autobiographical section not to defend himself but in order to defend the Gospel, which he received and was commissioned to preach (Gal. 1:12, 16). For, it was in the person of Paul the Apostle that the truth of the Gospel was being attacked and distorted.
The authenticity and authority of Paul’s apostleship were inseparable from the Gospel. The false teachers sought to attack Paul in order to destroy his Gospel.
If this has been merely a personal matter, it would have given no uneasiness to Paul to be reckoned an ordinary disciple. But when he saw that his doctrine was beginning to lose its weight and authority, he was not entitled to be silent. It became his duty to make a bold resistance. When satan does not venture openly to attack doctrine, his next stratagem is to diminish its influence by indirect attacks. Let us remember, then, that in the person of Paul the truth of the Gospel was assailed… (Calvin’s Commentaries, vol. 21, “Galatians,” p. 16).
In Galatians, Paul sets out to declare, demand, defend, define and direct the truth of the Gospel, the Gospel of Christian Freedom (2:5, 14, 16; 5:1). Thus, in light of the preceding discussion, the following outline of Galatians is offered:
I. The Gospel of Christian Freedom Declared (1:1-5)
II. The Gospel of Christian Freedom Demanded (1:6-10)
III. The Gospel of Christian Freedom Defended (1:11-2:21)
IV. The Gospel of Christian Freedom Defined (3:1-4:31)
V. The Gospel of Christian Freedom Directed (5:1-6:18)