I was composing a long reply, but had to restart my browser and lost it. I don't have time right now to type all of that again, so I'm basically just going to list some of my reasons and can give fuller argumentation later if anybody is interested. 1, 3, and 6 were basically the reasons for me leaving Presbyterianism.
1. The RPW. This was the main reason for me leaving Presbyterianism after having been in it for a few years, having attended OPC, PCA, and EPC congregations and having been a member of the OPC. There is no command, example, or inference for infant baptism in the NT. The specific passage that caused me to rethink was Acts 2. I think Acts 2 is a strong passage for credobaptism rather than pedobaptism. If we must pound the pulpit when reading Acts 2 and making arguments for baptism, it seems to me that the place to do it is at v. 41.
2. Also regarding Acts 2, isn't the promise here the promise of the Spirit? Isn't that the context of Peter's sermon, rather than an allusion to Gen 17?
3. If pedobaptism is true, then so is pedocommunion. From what I understand, the church, east and west, practiced pedocommunion, with Rome only abandoning it once the doctrine of transubstantiation was solidified. (It won't do to have junior vomiting out the "Body of the Lord.") I thought that it was easier to explain away the 1 Cor 11 argument against infant communion than it was to explain away the multitude of Baptist proof texts against infant baptism. If the "breaking of bread" in Acts 2 is a reference to the Lord's Supper, as some interpreters have held, then the passage teaches pedocommunion as well as pedobaptism if we accept the pedobaptist argument of "You and your children."
4. The vast majority of contemporary pedobaptists are inconsistent in adopting what are essentially baptistic views of church and state, which is more or less what the American revision of the WCF does. If we're going to have continuity with the OT when it comes to the subjects of baptism, then what grounds do we have for abandoning it when it comes to the relationship between the church and civil government? To use one of their favorite epithets against them, it is dispensationalism!!!
5. Without resorting to baptismal regeneration, how can a pedobaptist explain 1 Pet 3:21? In what sense is baptism "the answer of a good conscience toward God" to a "covenant child?"
6. Prior to Zwingli, who acknowledged his departure from the fathers on this subject, can we find any clear and unambiguous example of an argument for infant baptism in church history that doesn't involve baptismal regeneration?
7. Circumcision was more of a national marker than a spiritual one. To be sure, it had spiritual significance (which some Baptists wrongly deny.) But a boy was to be circumcised regardless of the faith of his parents. Reformed pedobaptists long ago abandoned that idea.
8. The household baptisms either seem to exclude infants or are inconclusive at best on the question of who was baptized.