Paul doesn’t always introduce himself as “an apostle” (1:1), but he did to the Corinthians (I Cor. 1:1; II Cor. 1:1) because they were doubting his apostleship I Cor. 9:1, 2). He wouldn’t have had to write things like that if they weren’t doubting his apostleship. And the Galatians must also have been doubting it, or he wouldn’t have to say it wasn’t “of men, neither by man.”
“Of men” means that men weren’t the source of his apostle-ship, as it does when the Lord asked Israel’s leaders if John’s baptism was “from heaven, or of men?” So Paul was saying men weren’t the source of his apostleship. But “neither by man” is talking about instrumentality, not source. That is, God didn’t use men to make him an apostle. Later he answers that by saying he didn’t get it from the 12 (Gal. 1:17).
Now the Corinthians doubted Paul’s apostleship because he didn’t look or sound like they thought an apostle should look and sound (II Cor. 10:10). But the Galatians doubted it because he had taught them what he taught the Romans, that we are saved by grace without the law (Rom. 3:21-28). But after he preached that for a few years, the Jews were alarmed, and got together in what is called “The Jerusalem Council” to decide if Paul was a legitimate apostle sent by God, and if his new message of grace was also from God (Acts 15:1-6).
Saved Jews like James recognized Paul’s apostleship was le-git (Acts 15:19), as did the leaders of the 12 (Gal. 2:9), and ever after that they quit telling Gentiles they had to keep the law to be saved. But unsaved Jews didn’t agree with the council, so they kept telling Gentiles that, including the Galatians. Those legalizers were probably telling them Paul’s apostleship was of men—the men of the Jerusalem Council, who mistakenly made him an apostle. But they didn’t make Paul an apostle, they just “saw” that he “was” one (Gal. 2:7).
They were probably also saying that God the Father would not have sent Paul to be an apostle to preach grace not law, for He’s the One who gave Moses the law. And they were probably also saying Christ wouldn’t have made him an apostle to preach grace not law, for Christ sent His apostles out to preach law, not grace (Mt. 23:1-3; 28:20)!
They may also have pointed out that Paul wasn’t made an apostle until after Christ had died, prompting Paul to say he was made an apostle by Christ and “God the Father, who raised Him from the dead” (1:4). He told the Romans that he received “grace and apostleship” from Christ after He rose from the dead (Rom. 1:3, 4), and sent him to preach grace to “all nations,” not law, like He sent the 12 to preach to “all nations” (Mt. 28:20).
Galatia was not a city, it was a “region” (Acts 16:6), and regions had many cities (cf. Deut. 3:4). Galatia was kind of like it’s own “country” (Acts 18:22, 23), and so may have had many “churches” (Gal. 1:2). If Paul was writing this letter of correction all of them, that shows how widespread the apostasy from grace was, even before Paul died.
But while Paul burned when people angered him like this (II Cor. 11:29), he offered the Galatians grace and peace (1:3), because that’s what God offers us when we anger Him. Paul’s flesh wanted to judge them and declare war on them for this apostasy, but offered them the opposite of judgment and war instead. This is similar to what we should do when men anger us, and to what God the Father did when He got good and mad at our sins. He sent His Son to die for them.
Galatians 1:4 says the Lord “gave Himself” for our sins to deliver us “from this present evil world,” not just the future evil world of hell. He did die to delivers us “from all iniquity” (Tit. 2:14), but also to “purify unto Himself a peculiar people, zealous of good works” now, in this present evil world. Today it is God’s grace that teaches us how to deny ungodliness (Tit. 2:11, 12).