Should the church expect all its members to be baptised? Does scripture suggest all those who joined the church had already been baptised? Here is Tom Schreiner in his seminal commentary on Romans (pp.306-7, 10), particularly with reference to Romans 6:3:
The reference to baptism is introduced as a designation for those who are believers in Christ. Since unbaptized Christians were virutally nonexistent, to refer to those who were baptized is another way of describing those who are Christians, those who have put their faith in Christ . Thus Paul is saying here that all Christians have participated in the death and burial of Christ, for all Christians had received baptism. To posit that the baptism mentioned here is simply metaphorical (Dunn 1988a: 311) or baptism in the Spirit (Lloyd-Jones 1973) rather than water baptism is incorrect. Moo (1991: 376) observes rightly that Paul normally uses the verb βαπτίζειν (baptizein, to baptize) to refer to water baptism (1 Cor. 1:13, 14, 15, 16 [twice], 17; 12:13; 15:29; Gal 3:27). Roman Christians would have inevitably thought of water baptism since it was the universal initiation rite for believers in Christ. Moreover, Paul probably loosely associated baptism with water and baptism by the Spirit (1 Cor. 12:13), since both of these occurred at conversion. Thus any attempt to distinguish between Spirit baptism and water baptism in the Pauline writings goes beyond what Paul himself wrote 
Paul’s intention in introducing baptism is not to emphasize “how we were buried with Christ, but to demonstrate that we were buried with Christ” (Moo 1991: 381). The emphasis is not on baptism as the means of God’s activity, although this is not excluded, but on the occassion of his work. Paul probably refers to baptism because it symbolizes dying and rising with Christ. Yet to separate baptism from other dimensions of the conversion experience is mistaken. For Paul baptism, faith, reception of the Spirit, repentance, and confession of Christ are one complex of events that all occur at conversion (cf. Fazekas 1996: 315-16; Dunn 1970: 145; Tappeiner 1977; Stein 1988: 116-26). Paul refers to believers as baptized because unbaptized Christians would be an anomaly.
- As Stuhlmacher (1994:91) says, “Child or infant baptism was not yet generally practiced in the church at the time of the New Testament”.
- Contra Stott (1994: 173), who goes beyond the evidence by distinguishing between water and Spirit baptism here. Stott is correct in saying that Paul was thinking of water baptism here, but it would never have occurred to Paul that baptism in water could be separated from baptism in the Spirit.