. . . what was the difference between the teaching of Paul and the teaching of the Judaizers? What was it that gave rise to the stupendous polemic of the Epistle to the Galatians? To the modern Church the difference would have seemed to be a mere theological subtlety. About many things the Judaizers were in perfect agreement with Paul. The Judaizers believed that Jesus was the Messiah; there is not a shadow of evidence that they objected to Paulâ€™s lofty view of the person of Christ. Without the slightest doubt, they believe that Jesus had really risen from the dead. They believed, moreover, that faith in Christ was necessary for salvation. But the trouble was, they believed that something else was also necessary; they believed that what Christ had done needed to be pieced out by the believerâ€™s own effort to keep the Law. From the modern point of view the difference would have seemed to be very slight. Paul as well as the Judaizers believed that the keeping of the law of God, in its deepest import, is inseparably connected with faith. The difference concerned only the logical â€“ not even, perhaps, the temporal â€“ order of three steps. Paul said that a man (1) first believes on Christ, (2) then is justified before God, (3) then immediately proceeds to keep Godâ€™s law. The Judaizers said that a man (1) believes on Christ and (2) keeps the law of God the best he can, and then (3) is justified. The difference would seem to modern â€œpracticalâ€ Christians to be a highly subtle and intangible matter, hardly worthy of consideration at all in view of the large measure of agreement in the practical realm. What a splendid cleaning up of the Gentile cities it would have been if the Judaizers had succeeded in extending to those cities the observance of the Mosaic law, even including the unfortunate ceremonial observances! Surely Paul ought to have made common cause with teachers who were so nearly in agreement with him . . .
Paul saw very clearly that the difference between the Judaizers and himself was the difference between two entirely distinct types of religion; it was the difference between a religion of merit and a religion of grace. If Christ provides only part of our salvation, leaving us to provide the rest, then we are still hopeless under the load of sin. . . . Such an attempt to piece out the work of Christ by our own merit, Paul saw clearly, is the very essence of unbelief; Christ will do everything or nothing, and the only hope is to throw ourselves unreservedly on His mercy and trust Him for all.
From J. Gresham Machen, Christianity and Liberalism (1923)