Do you not know that all of us who have been baptized into Christ Jesus were baptized into his death?
-Romans 6:3-
What he intimated in the last verse -- that Christ destroys sin in his people, he proves here by mentioning the effect of baptism, by which we are initiated into his faith; for it is beyond any question, that we put on Christ in baptism, and that we are baptized for this end -- that we may be one with him.
-John Calvin-
I do not think it stretches the words beyond ordinary use to say that Romans 6:3-4 describes the symbol of dying with Christ rather than the instrument of dying with Christ.... Now here's the analogy I would suggest to show that this language can be the language of symbol, not instrument: "All of us who have put on the ring of marriage have, by putting on this ring, forsaken all others to cleave only to our wives. Therefore by this ring I am united to my wife alone and dead to all others."... In that analogy, the vows stand for faith in Christ, and the ring stands for baptism. And the point is that we often talk this way. We often speak of the symbol as though it brings about what it only signifies.
-John Piper, October 1, 2000-
What is baptism's effect? Does baptism do anything? Many Evangelicals would say, “Absolutely not, baptism does nothing.” Piper echoes this sentiment. He and many of us, when we hear someone say something like, “Baptism now saves you (1Peter 3:21),” can only hear, “You are justified through the act of baptism.” This is clearly the mindset that Piper was arguing against when he spoke the words above. It is a good thing to argue against, because there are some that reject justification by grace, through faith and say that the act of baptism produces saving grace (ex opere operantis ).
But does baptism do nothing? Calvin clearly didn't think so. He had no problem saying that baptism initiates us into Christ's faith and that it joins us to Him. Does this mean that he rejected Sola Fide (justification through faith alone)? Possibly, but he did say that it initiates us “into His faith.” Plus, I am not about to start accusing Calvin of such things.
I personally do not know what baptism does; although, I have heard some very right-sounding suggestions. I am really only writing to point out the logical fallacy in Piper's argument. When he compares the act of baptism to a wedding ring he is begging the question in a very subtle way. He argues that baptism is only a symbol for the faith by which one is covenanted to God, and to do this he uses the symbol of another covenant. Clearly his argument can only be accepted if you already believe that baptism is only a symbol.
What would happen if we tweaked Piper's argument? What if we replaced the wedding ring with the wedding ceremony (after all baptism is more ceremonial than it is ring-like)? In that case, we must say that even if two people claim to have the devotion that the wedding ceremony corresponds to, they are not united until the ceremony. And if a man convinced a woman to have sex with him because “a marriage ceremony is only a symbol for the devotion that I already have toward you,” she will soon find his devotion fleeting. Perhaps we are united to Christ through Baptism and that ceremony obliges us to be devoted to Christ. Perhaps when a baptized person stops trusting Christ he is being unfaithful to the union he entered through baptism. And perhaps all this can be said without undermining Sola Fide. Of course, that's just a possibility.