I've posted on Baptism, before. You can find that, here. I am a credo-baptist, however, unlike many Baptists, I do not think that someone who was baptized as an infant has a baptism that is necessarily invalid. I think we have an example that makes this consistent with the theology we already have in place. You can listen to a short exposition of that position, here, by John Piper.
A Cognate:
If you took the time to click on the Piper link, what you heard was a description of whether we should baptize those who are mentally challenged to the point that they cannot make a public profession. He argues that we should baptize these folks on the basis of a perception that we can gain from that individual. He then argues that we should probably not baptize these people if we gain no perception from the individual.
I mostly agree with Piper, here. I think he has focused on God's grace and also defended the sanctity of Baptism in a very meaningful way. Therefore, I also agree with him that the Baptism of these non-professors would be valid.
Infant Baptism?
Okay, so, what about infant baptisms, are all infant baptisms valid? NO, they most certainly are not. Okay, I am sure that I've lost a bunch of folks at this point, but bear with me. I, as a Baptist, don't remember a single time in my life where I did not love God. That's not to say that there was no specific moment where I repented of my sins and professed myself to be a follower of Christ. There was definitely such a time, and it was on December 18th, 1995. I was six years old. But, thinking in terms of a Calvinistic soteriology, I do not believe that I lacked faith in God and his promises before that point, and I do think that God regenerated my soul before that point and brought me to that faith.
If I had been baptized before my profession, given those details, I would readily accept them as valid. The presence of that regeneration, though prior to my profession, would be evident in a way that was similar to what we have seen in the discussion of the mentally disabled, above. If I am going to count the baptism of the mentally disabled as valid without a clearly articulated profession, it is only consistent that I grant the same to those infants who were baptized and who showed signs of God's regenerative work before articulating a confession.
Now, in line with what Piper talked about with those who showed no signs of regeneration... if a person who was baptized as an infant do not reflect in their lives the work of regeneration, and live out a rebellion against God without ever having done so, and then they repent and come to faith. I would say that when that person was baptized there was no evidence of the work of redemption and therefore no evidence that God had worked regeneration in their souls. This person was an improper candidate for baptism as an infant.
Why not baptize everyone as infants?
As a Baptist, I believe that the form of baptism prescribed for believers is upon profession of faith. I respect a lot of my paedo-baptist brothers and I cherish them. However, I also believe that baptism by immersion is what is prescribed in Scriptures, and I would readily affirm that a baptism is not invalidated simply because it was not by immersion (Sam Waldron, who wrote an exposition of the LBCF 1689 that I have reviewed a few chapters of, agrees.) So, though a baptism not done by immersion would be irregular, it's still a valid baptism. I think in the case of these infant baptisms we have the same situation.
Conclusion
A pastor or group of elders at a church is unlikely to know the circumstances of God's regenerative work in a person. Thus, I put the responsibility on the believer to determine their baptismal candidacy as it relates to their infant baptism. I believe this very short description matches a sense in which a person can both hold to credo-baptism and also affirm that infant baptism can be valid, but irregular.
