Galatians 3:27-29 (ESV) – For as many of you as were baptized into Christ have put on Christ.  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.  And if you are Christ’s, then you are Abraham’s offspring, heirs according to promise.
It is possible for a person to be the recipient of believer’s baptism and not be a believer. The New Testament talks much about false faith – faith that is not a genuine faith in Jesus Christ. Such false believers are often baptized (See Acts 8:9-24). They are either fooling themselves or practising deceit.
Paul is not speaking here about such people. He is speaking to those who have truly put on Christ and therefore have been truly baptized into Christ. It is unlikely that Paul knew every Christian in the Galatian churches that he is intending this Letter to go to. But even though he did not know them he knew this – they had all been baptized. Conversion and baptism are inseparable in the New Testament.
If you said to the Apostle Paul “What should we do about believers who are not yet baptized”, he would not be able to process the question. A non-baptized believer is simply not, in Paul’s mind (and the ‘mind’ of the whole New Testament), a possibility. And if you are a believer who has not been baptized you should be. It does not contribute to you being saved, but it is an act of obedience and brings with it, blessing from Christ.
Paul is not saying in the opening words of this verse (27) that believers do not receive Christ until the have been baptized. Neither is he saying that baptism somehow deploys the Spirit of Christ through baptism.
He is saying that all believers have been baptized into Christ and nothing more is necessary. We must not forget the context of this whole letter. Salvation is not faith plus anything. It is faith alone in Christ alone. It is not a matter of ethnicity, social standing or gender – it is a matter of faith in Christ. Keeping the Law does not add to your relationship with Christ – all who have been baptized are in Christ. They have put on Christ. To add to this is to deny the Gospel. Belonging to Christ makes you a descendant of Abraham – not keeping the Law, not gender and not social class. What a glory the Gospel is.