As we see in the case of the Judaizers below, Paul saw here the differences between two entirely distinct types of religion; one was a religion of merit and the other a religion of grace. He knew that if Christ provides only a part of our salvation, leaving us to provide the rest, then we are still hopeless under the load of sin. According to J. Gresham Machen:
The beginnings of Christianity constitute a fairly definite historical phenomenon. The Christian movement originated a few days after the death of Jesus of Nazareth. It is doubtful whether anything that preceded the death of Jesus can be called Christianity. At any rate, if Christianity existed before that event, it was Christianity only in a preliminary stage. The name originated after the death of Jesus, and the thing itself was also something new. Evidently there was an important new beginning among the disciples of Jesus in Jerusalem after the crucifixion. At that time is to be placed the beginning of the remarkable movement which spread out from Jerusalem into the Gentile world—the movement which is called Christianity.
About the early stages of this movement definite historical information has been preserved in the Epistles of Paul, which are regarded by all serious historians as genuine products of the first Christian generation. The writer of the Epistles had been in direct communication with those intimate friends of Jesus who had begun the Christian movement in Jerusalem, and in the Epistles he makes it abundantly plain what the fundamental character of the movement was. But if any one fact is clear, on the basis of this evidence, it is that the Christian movement at its inception was not just a way of life in the modern sense, but a way of life founded upon a message. It was based, not upon mere feeling, not upon a mere program of work, but upon an account of facts. In other words it was based upon doctrine.
Certainly with regard to Paul himself there should be no debate; Paul certainly was not indifferent to doctrine; on the contrary, doctrine was the very basis of his life. His devotion to doctrine did not, it is true, make him incapable of a magnificent tolerance. One notable example of such tolerance is to be found during his imprisonment at Rome, as attested by the Epistle to the Philippians. Apparently certain Christian teachers at Rome had been jealous of Paul’s greatness. As long as he had been at liberty they had been obliged to take a secondary place; but now that he was in prison, they seized the supremacy. They sought to raise up affliction for Paul in his bonds; they preached Christ even of envy and strife. In short, the rival preachers made of the preaching of the gospel a means to the gratification of low personal ambition; it seems to have been about as mean a piece of business as could well be conceived. But Paul was not disturbed. ‘Whether in presence, or in truth,’ he said, ‘Christ is preached; and I therein do rejoice, yea, and will rejoice’ (Phil. i. 18). The way in which the preaching was being carried on was wrong, but the message itself was true; and Paul was far more interested in the content of the message than in the manner of its presentation. It is impossible to conceive a finer piece of broad-minded tolerance.
But the tolerance of Paul was not indiscriminate. He displayed no tolerance, for example, in Galatia. There, too, there were rival preachers. But Paul had no tolerance for them. ‘But though we,’ he said, ‘or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be accursed’ (Gal. i. 8). What is the reason for the difference in the apostle’s attitude in the two cases? What is the reason for the broad tolerance in Rome, and the fierce anathemas in Galatia? The answer is perfectly plain. In Rome, Paul was tolerant, because there the content of the message that was being proclaimed by the rival teachers was true; in Galatia he was intolerant, because there the content of the rival message was false … His opposition was based altogether upon the falsity of their teaching; they were substituting for the one true gospel a false gospel which was no gospel at all. It never occurred to Paul that a gospel might be true for one man and not for another; the blight of pragmatism had never fallen upon his soul. Paul was convinced of the objective truth of the gospel message, and devotion to that truth was the great passion of his life. Christianity for Paul was not only a life, but also a doctrine, and logically the doctrine came first. (“Christianity and Liberalism”)