A sign.

Baptism - the sign of entry of our saving union with Christ in his death and resurrection, baptism is the sign or 'sacrament'. But baptism is not the means of union. This is clear from a comparison between baptism and circumcision. Baptism is the sign of entry into the new covenant, as circumcision was the sign of entry into the old covenant. Now circumcision is defined by Paul as 'a sign or seal of the righteousness which he [sc. Abraham] had "by faith" while he was still uncircumcised' (Rom. 4:11). First Abraham received justification by faith. Then he received circumcision as a sign. It is by faith that we are joined to Christ and so justified and of this faith Abraham and David are the best Old Testament examples.

-- John R.W. Stott