Gal 3:5-13 reads:
Well then, does God supply you with the Spirit and work miracles among you by your doing the works of the law, or by your believing what you heard? Just as Abraham "believed God, and it was reckoned to him as righteousness," so, you see, those who believe are the descendants of Abraham. And the scripture, foreseeing that God would justify the Gentiles by faith, declared the gospel beforehand to Abraham, saying, "All the Gentiles shall be blessed in you." For this reason, those who believe are blessed with Abraham who believed. For all who rely on the works of the law are under a curse; for it is written, "Cursed is everyone who does not observe and obey all the things written in the book of the law." Now it is evident that no one is justified before God by the law; for "The one who is righteous will live by faith." But the law does not rest on faith; on the contrary, "Whoever does the works of the law will live by them." Christ redeemed us from the curse of the law by becoming a curse for us-- for it is written, "Cursed is everyone who hangs on a tree"--in order that in Christ Jesus the blessing of Abraham might come to the Gentiles, so that we might receive the promise of the Spirit through faith. (NRSV)
There are a number of important things to note about this pericope:
Firstly, in v. 5, with respect to the reference to one receiving the "Spirit," the New Testament often uses "Spirit" in conjunction with the intrinsic, transformational changes in the individual, so a transformation, not a mere forensic declaration, is in view in this text. That Paul, too, understood this transformational nature of the Spirit and the believer's receiving thereof in 1 Cor 12:4-11 where he applies such language to the spiritual gifts, not a mere forensic imputation:
Now there are diversities of gifts, but the same Spirit. And there are differences of administrations, but the same Lord. And there are diversities of operations, but it is the same God which worketh all in all. But the manifestation of the Spirit is given to every man to profit withal. For to one is given by the Spirit the word of wisdom; to another the word of knowledge by the same Spirit; To another faith by the same Spirit; to another the gifts of healing by the same Spirit; To another the working of miracles; to another prophecy; to another discerning of spirits; to another divers kinds of tongues; to another the interpretation of tongues: But all these worketh that one and the selfsame Spirit, dividing to every man severally as he will.
Secondly, that Paul is speaking of justification, not sanctification merely, can be seen in Gal 3:14 where Paul writes "ἵνα εἰς τὰ ἔθνη ἡ εὐλογία τοῦ Ἀβραὰμ γένηται ἐν Χριστῷ Ἰησοῦ" ("in order that, in Christ Jesus, the blessing of Abraham might come to the Gentiles"). As seen in vv.8-9, the “blessing” of Abraham is his being justified in Gen 12 (see The "Blessing" of Abraham in Genesis 12: Evidence of Progressive Justification; cf. Does Genesis 15:6 prove Reformed Soteriology?).
Thirdly, further reinforcing that progressive justification is in view is seen in the latter part of Gal 3:14, "ἵνα τὴν ἐπαγγελίαν τοῦ πνεύματος λάβωμεν διὰ τῆς πίστεως" ("in order that we might receive the spirit through faith"). In this verse, we see the interchange between the Spirit and justification. Paul could easily have replaced "promise of the Spirit" with "justification" and it would mean the same thing, especially in light of his focus on justification in vv. 6-13.
Another passage in Galatians that supports progressive justification is Gal 5:1-6:
For freedom Christ has set us free. Stand firm, therefore, and do not submit again to a yoke of slavery. Listen! I, Paul, am telling you that if you let yourselves be circumcised, Christ will be of no benefit to you. Once again I testify to every man who lets himself be circumcised that he is obliged to obey the entire law. You who want to be justified by the law have cut yourselves off from Christ; you have fallen away from grace. For through the Spirit, by faith, we eagerly wait for the hope of righteousness. For in Christ Jesus neither circumcision nor uncircumcision counts for anything; the only thing that counts is faith working through love.
In this text, Paul sets up a context that considers Christians "free" (initially justified), especially in light of vv.1-2. However, that Paul's theology here also incorporates progressive justification can be seen in the following lines of evidence:
Firstly, in v. 4, he makes a very "non-Protestant" statement: "You who want to be justified by the law have cut yourselves off from Christ; you have fallen away from grace." From the perspective of many Protestant theologies, Paul should have wrote, "You who want to be sanctified by the law . . ." That Paul uses justified instead of sanctified shows that not only that various Protestant understandings of sanctification are in view in this verse, but also that Paul believes that those spoken of in vv.1-2, those made "free" by Christ, can, by following the "Galatian heresy," fall away from Christ, not simply suffer fatherly discipline for sin while remaining eternally secure. Furthermore, Paul says they can be "cut off" (per BDAG, with reference to Gal 5:4, καταργεω means "to cause the release of someone from an obligation (one has nothing more to do with it), be discharged, be released") from "grace," so "justification" is in view, not "sanctification."
Secondly, evidence of the progressive nature of justification can be seen in the Greek of v. 4. "Justified" is δικαιοῦσθε, the second person indicative present passive of δικαιοω, denoting an on-going concern with justification.
Thirdly, Paul in vv. 13-16, writes:
For you were called to freedom, brothers and sisters; only do not use your freedom as an opportunity for self-indulgence, but through love become slaves to one another. For the whole law is summed up in a single commandment, "You shall love your neighbor as yourself." If, however, you bite and devour one another, take care that you are not consumed by one another. Live by the Spirit, I say, and do not gratify the desires of the flesh. (NRSV)
In this pericope, Paul continues the theme of "freedom" as well as continuing the ominous theme that one can be "cut off" from Christ. Furthermore, he is condemning not only a retreat to the ceremonial law, but a neglect of the moral law. By continuing in this rebellious state, Paul warns them thusly:
Envyings, murders, drunkenness, revellings, and such like: of the which I tell you before, as I have also told you in time past, that they which do such things shall not inherit the kingdom of God. (Gal 5:21)
For he that soweth to his flesh shall of the flesh reap corruption; but he that soweth to the Spirit shall of the Spirit reap life everlasting. (Gal 6:8)
In Gal 6:8, Paul sets up “eternal life” (ζωὴν αἰώνιον) with “corruption,” or, as the Greek φθορά can also denote, “(eschatological) destruction.” Indeed, referencing Gal 6:8, BDAG offers the following definition of φθορά:
5. total destruction of an entity, destruction in the last days Gal 6:8 (opp. ζωὴ αἰώνιος). ἐν τῇ φθορᾷ αὐτῶν καὶ φθαρήσονται when they (the dumb animals) are destroyed in the coming end of the world, these (the false teachers), too, will be destroyed (so BWeiss, Kühl, JMayor, Windisch, Knopf, Vrede) 2 Pt 2:12b.—DELG s.v. θείρω. M-M. TW. Sv.
Such proves that Paul is not worried about their on-going sanctification merely, but also their justification, one they can actually lose and fall under eschatological condemnation.
All of these considerations prove that Paul, in Galatians, supports the doctrine of progressive justification as well as the doctrine that a truly justified individual can lose their justification. All of these are very “Mormon” doctrines, but not Protestant, especially Calvinistic. Again, we see that, notwithstanding claims to the contrary, if any faith can qualify as being that of "Biblical Christianity," it is The Church of Jesus Christ of Latter-day Saints.