Paul's letter to the Galatians addresses the nature of the Law and its role in the salvation of the Gentiles. The blessed apostle makes the point over and over that the Law is a relic of the past for the Galatians, because in Christ they have inherited the promise of blessing made to Abraham. Paul's argument depends on the notions of participation in Christ, union with Christ, etc.
Abraham was given a promise, which Paul interprets as the promise of the Spirit (Gal 3.14). For Paul the promise of blessing is a union with God and a fellowship with the Holy Trinity through the Spirit. The promise is nothing less than the embodiment of the image and likeness of God and fellowship with the Creator, the intention of God for humanity from the beginning (cf. Gen 1.26-7).
This promise was given, says Paul, to Abraham and his singular offspring (Gal 3.16). This offspring, this son of Abraham, is Jesus Christ. Peter too speaks of Christ's having received the Spirit in his sermon at Pentecost (Acts 2.33). Now here comes the critical point: Christ received the promised Spirit, but this isn't itself any good news unless I can somehow unite myself with Christ and gain the Spirit as well. And this is precisely Paul's gospel: through our union with Christ, we become adopted children of God, just as Christ is a natural Son, and we too get the Holy Spirit. This is what salvation consists in: adoption through union with the Son.
This is what Paul says: God sent his Son, born of a woman, born under the law, in order to redeem those who were under the law, so that we might receive adoption as children. And because you are children, God has sent the Spirit of his Son into our hearts, crying, "Abba! Father!" So you are no longer a slave but a child, and if a child then also an heir, through God (4.4-7).
Now how does this happen? Paul certainly emphasizes that the gospel is good news for those who believe, but he is also clear that baptism plays an important role: . . . in Christ Jesus you are all children of God through faith. As many of you as were baptized into Christ have clothed yourselves with Christ (3.26-7). It is by being baptized into Christ that we are clothed with him, that we are united with him and thus benefit from the adoption as sons. Through baptism we receive the promised Holy Spirit through our union with Christ.
It is evident from the logic of Paul's argument that baptism is the mode through which we are united to Christ. This is what he says elsewhere in Romans, too, when he says that in baptism we have been united with him in a death like his (Rom 6.5).
I can't understand some persons in large evangelical free churches who have believed for a long time and yet have never been baptized. Likewise I don't understand persons who claim to be Christians, who've been in the church their whole lives, and yet don't seek baptism. Paul emphasizes that our union with Christ is through our baptism; our salvation is closely connected to baptism. Some persons may have the impression that baptism is a ritual only for the pure, or for the serious, for the worthy. This impression is woefully ungrounded and mistaken, however, since it is precisely because you are unworthy that you ought to run to get baptized. Through baptism you are united to Christ, you receive the Holy Spirit, and are thus made worthy.