“It is clear that in Paul’s own mind the case of Abraham (receiving righteousness through faith) provides an obvious parallel to that of the Galatians (receiving the Spirit through hearing and believing the gospel and not by keeping the law). In other words, Paul takes it for granted that Abraham’s being justified by faith proves that the Galatians must have received the Spirit by faith also; and this argument from Scripture falls to the ground unless the reception of the Spirit is in some sense equated with justification. For if this were not so, it could be objected that even though Abraham was indeed justified by faith, it does not necessarily follow that reception of the Spirit also has to be dependent on faith; conceivably while justification is by faith the gift of the Spirit could be conditioned on works. We may take it, then, that Paul conceives of receiving the Spirit in such close connection with justification that the two can be regarded in some sense as synonymous, so that in the Galatians’ receiving the Spirit their justification was also involved.
Thus, just as in the previous passage (2.15-21) Paul interpreted his own conversion experiences in terms of justification by faith, so also in the present passage (3.1-6) the Galatians’ initiatory experience of receiving the Spirit is regarded as at least involving justification by faith, if not being totally synonymous with it. This again shows that to Paul justification stands at the inception of the Christian life as an integral part of the Christian experience.”
Furng, R.K. Galatians, pg 136-136 in regard to Gal 3.6