Crossway has made available Doug Moo’s essay, “Justification in Galatians” (PDF) from
Understanding the Times: New Testament Studies in the 21st Century: Essays in Honor of D. A. Carson on the Occasion of His 65th Birthday, ed. Köstenberger and Yarbrough.

An excerpt:

Paul’s teaching on justification in Galatians strongly endorses the traditional Reformation emphasis on justification by faith alone.

In contrast to some recent reconfigurations of this doctrine, the Reformers did not mean by this teaching that a person gains only initial entrance into the state of salvation by faith alone—the ultimate verdict being based on faith plus works.

They intended to assert that the eschatological gift of justification, at whatever “time” or in however many stages it might be manifested, came by faith alone.

Paul seems to be saying just this in Galatians. Faith is the means not only of entering into relationship with God but also of maintaining that relationship and of confirming that relationship on the day of judgment.

Of course, it is not faith in itself that has this power; it is because faith connects the believer to Christ, in whose vindication (see 1 Tim. 3:16) the believer shares.

My brief overview confirms those who find a monergism in Paul’s teaching about salvation that stands in contrast to the synergism of covenant nomism. Justification, not only in its initial phase, but in its totality, is sola fide—and, though it has not been a focus of this study, in light of Galatians 2:21 and 5:4, sola gratia also.