The Significance of Baptism
Baptism may be compared to a wedding ceremony. A couple may be very much in love with each other, but it is only through marriage that they are pronounced as husband and wife. The wedding ceremony, with exchange of vows and symbols of union establishes them as belonging to each other and becoming one. Similarly, baptism pronounces the baptized publicly as united to Christ.
However, in a community that desacralizes the sanctity of marriage, the significance of a wedding ceremony soon fades. If conjugal rights and all rights of marriage are not made dependent on the act of marriage, then marriage loses its significance. Similarly, a church that doesn't provide baptism as a condition desacralizes the divine commandment of baptism.
This is one reason why I believe annunciating baptism as precondition for participation in communion is quite justified. A married couple may have communion with each other daily, but this communion flows from the single act of marital union in their nuptial ceremony. Similarly, a believer may partake of communion daily as remembrance of Christ's death. However, that remembrance flows from his/her single act of union with the Lord in the baptism of His death, burial, and resurrection. In the sameway that marriage is once for all, baptism is also one.