Yesterday, Shane wrote something affirming the shared efficacy of baptism within the Reformed faith for infants and adults. I want to highlight a quote from John Calvin that I believe supports this position.
In his section on the sacraments, Calvin deals with the consequences "if infants were excluded from baptism" (Q337). Calvin is clear that "their privileges" would be diminished. He then expands on this idea in the next question,
338 M. This is your opinion then — As much as God, under the Old Testament, that he might show himself to be the Father of little children, commanded the promise of salvation to be engraved on their bodies, by a visible sign, it would be a reproach, if believers, after the coming of Christ, should have a less confirmation. When the same promise, which was formerly given to the Fathers, is ordained for us in these days, when God exhibits to us in Christ a clearer manifestation of his goodness.
C. So I think. Besides, as it is sufficiently evident, that the power and substance (so to speak) of baptism, are common to infants, then if the sign is denied them, which is inferior to the substance, a manifest injury will be done them.
Reformed believers should not shrink back from this language. For Calvin, baptism is at least a "promise of salvation" and this is in its application to children. Calvin goes further and says that the "power and substance" of baptism is "common to infants." Without delving into the nature of the "power and substance," Reformed Christians should cling to the idea that it is now "common" to such a degree that "manifest injury" is done when we withhold baptism.
Calvin argues this way because the sign "is inferior" to the substance. Implicit in his thought is a connection that depriving the sign can deprive — so to speak — the substance. Calvin is not saying baptism is essential for salvation. But neither is it inconsequential to withhold. In this regard, Calvin treats baptism as more than a nebulous change of labels (e.g. heathen to external church member) but as a true means of grace.