Anyone (like me) who was confused about what exactly Piper was thinking about infant baptism recently should read chapter 18 of Brothers We Are Not Professionals. He lays out his position on believers baptism. Here are a couple quotes:
Here’s how my thought has progressed. There have been three stages–not unlike childhood, adolescence, and (I hope) maturity.
First, I saw that every baptism recorded in the Bible was the baptism of a person who had professed faith in Christ…But I gradually came to see that these observations were only suggestive, not compelling. The fact that no infant baptisms are recorded does not prove that there weren’t any…But Colossians 2:12 and 1 Peter 3:21 seemed to me to be problematic for the paedobaptist view…It seems therefore, that Paul is saying that baptism is an expression of the faith of the person being baptized. I did not see how an infant could properly receive this ordinance as an expression of his or her faith…
(aside concerning 1 Peter 3:21)
This text frightens many Baptists away because it seems to come close to the Roman Catholic notion that the rite, in and of itself, saves (baptismal regeneration). But in fleeing from this text, we throw away a powerful argument for believer baptism. For as J.D.G. Dunn says, ‘1 Peter 3:21 is the nearest approach to a definition of baptism that the NT affords.’
According to Peter, baptism is “an appeal to God.” That is, baptism is the cry of faith to God. In that sense and to that degree, it is part of God’s means of salvation. This should not scare us off any more than the sentence, ‘If you confess with your mouth that Jesus is Lord…you will be saved” (Rom. 10:9). The movement of the lips in the air and the movement of the body in water save only in the sense that they give expression to the single justifying act, namely, faith (Rom. 3:28). Baptism is the outward appeal of faith to God in the heart.
…Since then I have been shown by a long succession of arguments in my church membership classes that even these texts leave open the remote possibility that an infant can be baptized on the strength of its parents’ faith and in hope of its own eventual “confirmation.” The argument says it is possible that those passages from Colossians and 1 Peter have relevance only for the missionary setting where adults are being converted and baptized. If Paul and Peter had addressed the issue of infants born into Christian homes, maybe they would have sounded like good Presbyterians…I doubt it. For there is now a third stage of reasoning in favor of believer baptism. There is a grand Biblical and Baptist response to the Heidelberg Catechism’s answer to question 74 as to whether infants are to be baptized”…(see Heidelberg Catechism)…”In other words, the justification of infant baptism in the Reformed churches hangs on the fact that baptism is the New Testament counterpart of circumcision.
There is in fact an important continuity between the signs of circumcision and baptism, but the Presbyterian representatives of Reformed theology seem to have undervalued the discontinuity…I am a Baptist because I believe that on this score we honor both the continuity and discontinuity between Israel and the church and between their respective covenant signs…This implies that entry into the old covenant people of God was by physical birth, and entry into the new covenant people of God is by spiritual birth. It would seem to follow then, that the sign of the covenant would reflect this change and would be administered to those who give evidence of spiritual birth.