I noted recently over on the Gospel Coalition website/blog they posted 2 answers to the question of what someone who was baptised as an infant, but came to faith as an adult, ought to do. The two answers were entirely predictably, indeed there has been little new in the paedo- vs. credo- baptist argument for a long time.
I want to outline some thoughts on baptism first, which will lead you to understand my mediating position.
Firstly, I take it that baptism is an outward sign of an inward reality. That inward reality is regeneration. I am happy, depending on context, to call it either a sacrament, or an ordinance, or even just a command of the Lord Jesus. I think that the normal, and perhaps normative, pattern of the New Testament is that believers, and ordinarily only believers, ought to be baptised. The main reason I think this is that the points of continuity *and* discontinuity with the Old Testament practice of circumcision strongly point me in this theological direction. In the OT, circumcision was the physical sign of entry into the covenant community. Entry into that community was by physical descent, generally, except in the case of converts, who became not merely converts to the Jewish faith, but members of the Jewish nation. There was an overlap of national and religious identity. In the NT, physical descent counts for nothing. Entry into the covenant community is by repentance and faith. The NT examples of people entering the new covenant community give us a pattern of people repenting, believing, and being subsequently baptised as the outward sign of that inward reality.
So far, so good, at least for the credo-baptist position. But I suspect that many baptists really have this attitude towards the question of (re-)baptising those baptised as infants - does it count? was it valid? And I think that is asking entirely the wrong question. There is something incredibly legalistic about such a question, because we all ought to know, if the thief on the cross teaches us anything, that baptism itself neither saves, nor is necessary for salvation. It is ordinarily the pattern that those who exhibit the evidences of grace get baptised, but that baptism itself does not constitute the salvific grace.
So on one count, I consider the idea that we need to 'do baptism right' otherwise it 'doesn't count', as to be somewhat misguided. On the second account, those of a baptistic tradition tend to identify baptism as a public individual pronouncement that one has chosen to follow Christ. There is something to that, but more than that I would argue that baptism is not so much something one does, but something that happens to you.
Just as baptism reflects the work of God in election and conversion, which was through no merit of our own, it represents our dying and living with Christ, a union we did nothing to effect, and so we are immersed in water, we do not immerse ourselves. Baptism happens to us. And so far as that is the case, I think it relatively indifferent then, that in the case of those who were baptised as infants, that the order got 'mixed up' so to speak.
See, where the outward sign preceded the inward reality, but the inward reality now matches the prior outward sign, then the sign 'does its job' - it signifies that this person belongs to the new covenant community through the washing of sins. To re-do that implies something deficient, and yet it seems to me that nothing is deficient. Paedo-baptism is, in this view, abnormal but not defective.