In Galatians 3 (and in the entire letter to Galatians) the context is an encouragement to start in the spirit and continue in the spirit. The whole division of Galatians. 3:1 – 4:7 (which is one logical division) speaks of the work of the spirit, and verses 3:26-29 about how we got into Christ to get qualified for the promise made to Abraham also speaks of the work of the spirit.
26 For you are all sons of God through faith in Christ Jesus. 27 For as many of you as were baptized into Christ have put on Christ. 28 There is neither Jew nor Greek, there is neither slave nor free, there is neither male nor female; for you are all one in Christ Jesus. 29 And if you are Christ’s, then you are Abraham’s seed, and heirs according to the promise.
Therefore, v. 27 “for all of you who were baptized into Christ have clothed yourselves with Christ” has to also be speaking of the Holy Spirit baptizing us into Christ.
Let’s consider anything other than trusting in Jesus as a requirement for inclusion in Christ. Let’s take water baptism as an example. If water baptism was a requirement for salvation, that would actually be antithetical to the main thrust of the epistle to Galatians, since that would contradict the main thesis of salvation and fruitfulness by faith alone apart from any “qualifying” works that we can produce with our efforts. In Galatian churches of that day, the “plus something” idea that was gaining currency was circumcision. Notice how throughout the entire epistle Paul vehemently opposes the idea of any requirement for receiving the promise as an addition to simply placing trust in the finished work of Christ.
Paul caps the discussion in Galatians by saying this:
Neither circumcision nor uncircumcision means anything; what counts is the new creation.