Confessional Reformed/Presbyterian churches don’t rebaptize a Christian who comes from another church to join theirs. The Westminster Confession of Faith (28:7) says “the sacrament of baptism is but once to be administered to any person.” For example, if a person was baptized in a Roman Catholic, Methodist, Brethren, or Baptist church, he or she would not have to be baptized again to join a Reformed/Presbyterian church.
Well, there are quite a few historical and biblical answers to the question. I don’t have the space here to discuss how the Reformers spoke against the Anabaptists who began rebaptizing Christians during and after the Reformation. You can read Luther’s 1528 treatise, “Concerning Rebaptism” for more information on this. The (short) historical answer to the above question (Why not?) is simply this: because we’re not Anabaptists!
At the heart of the biblical answer is the fact that baptism is primarily God’s sign and seal of his covenant of grace rather than an action we perform when we believe. If a person is baptized in the name of the Triune God, according to the command of Christ, it’s an objective sign that doesn’t need to be repeated – just like circumcision in the Old Covenant didn’t need to be repeated. Speaking covenantally, John Calvin said, “however the covenant might be violated by them [wayward Jews in the OT], the symbol of the covenant remained ever firm and inviolable by virtue of the Lord’s institution” (Institutes, IV.XV.17).
Robert Shaw, a 19th Century Presbyterian pastor, explained it like this:
“Baptism is not to be administered to any person oftener than once. This is plain from the nature of the ordinance. It is a solemn admission of the person baptized as a member of the visible Church; and though those that ‘walk disorderly’ are to be cast out, yet there is no hint in Scripture, that, when re-admitted, they are to be baptized again. The thing signified by baptism cannot be repeated, and the engagements come under can never be disannulled” (Exposition of the Westminster Confession of Faith, p. 370).
Of course, we should always be prepared to profess our faith before God’s people (Ps. 22:22) and we should continually repent of our sins (Ps 51), but we don’t need to be baptized more than once because it is God’s covenant sign and seal of the covenant of grace. Because his covenant promises never change and because he is faithful, baptism is something Christians only need to undergo once. (If baptism depended on my faith, I’d have to be baptized several times a year since my faith waxes and wanes!)
Baptism is a “one time” sacrament that benefits us our whole life. When we stumble, baptism reminds us of God’s promises and Christ’s shed blood. We flee to the Lord with repentant faith, plead his promises, and rejoice that his blood covers all sins. As Luther puts it in the above mentioned treatise, there is always something lacking in our faith. But there is never anything lacking in our baptism because it is God’s covenant sign and seal. That’s a short answer to the question of why Reformed and Presbyterian churches don’t practice rebaptism.