According to both Holy Scripture and the Westminster Standards, rebaptism is a sin. It is a transgression of the Law of God. For the Decalogue commands that God be worshipped only in the authorized way — and not through any 'graven images' (such as rebaptism) contrary to His revealed will.
In Old Testament times, bodily circumcision is unrepeatable –and recircumcision was and is impossible. Deut 10:16 & 30:6 and Jeremiah 4:4 & 9:25-26. Because circumcision has now been replaced by baptism, the latter too is unrepeatable — and rebaptism impossible. Romans 4:11-25 & 6:1-5; Galatians 3:6-29; Colossians 2:11-13.
Only unitarians and apostates practised 'rebaptism' in the apostolic age. Mark 7:3-8; Acts 19:1-3; First Corinthians 11:18f & 15:29. To the True Visible Church of the Triune God, there was only one baptism — trinitarian, life-long, and unrepeatable. Matthew 28:19f; Mark 16:15f; Romans 6:3-23; Ephesians 4:4-6; Colossians 2:6-16.
Hebrews 6:1-6 implies that those who get themselves rebaptized, recrucify Christ. For it commands: "Do not again lay down…the doctrine of baptisms…. They crucify to themselves the Son of God afresh!" See Francis Nigel Lee: Rebaptism Impossible.
The Westminster Confession of Faith 28:7 declares that "the sacrament of baptism is but once to be administered to any person. Titus 3:5."
The Westminster Larger Catechism 108 & 112 rightly insists that the Second Commandment requires the proper "receiving of the sacraments. Matthew 28:19." Indeed, the Third Commandment requires that the "sacraments…be holily and reverently used…by an holy profession."
Consequently, the Westminster Larger Catechism 177 also requires "that baptism is to be administered but once with water — to be a sign and seal of our regeneration and ingrafting into Christ. Matthew 3:11 & Galatians 3:27."
According to both Holy Scripture and the Westminster Standards, being unbaptized is a sin! Omitting to have also one's baby baptized, is to break the Law of God.
God solemnly warns us not to neglect getting the sacrament of initiation administered to our own babies. See Francis Nigel Lee: Have You Been Neglecting Your Baby? On the Serious Consequences of Withholding Baptism from the Infants of Christians.
In Genesis 17:10-14, God demands that all covenant babies "must needs" receive the sign of the covenant — or be "cut off" from God's people for "breach" of the covenant! Commented Calvin: "As God adopts the infant son in the person of his father, so when the father repudiates such a benefit — the infant is said to cut himself off from the Church…. God indeed will not acknowledge those as among His people, who will not bear the mark and token of adoption!"
In Exodus 4:24-26, God sought to kill Moses — for neglecting to give the sign of the covenant to his infant child. Commented Calvin: "Let us then learn from hence to use reverently the sacraments, which are the seals of God's grace — lest He should severely avenge our despisal of them!"
In Exodus 12:24-48, God debars from the second sacrament all adults whose infants still lack the first sacrament. Comments Calvin: "They should also teach their children…, for doctrine may justly be called the life of sacraments…. The Paschal Lamb corresponds to the Holy Supper…. None but the initiated were admitted…. From the analogy between the Holy Supper and the Passover, this Law remains in force now!"
In Ezekiel 44:7 — a foreshadowing of the New Testament Church –God rebukes those who have received the sacrament of initiation for bringing those who have not, to worship in His presence. Declares God: "You have brought into My sanctuary strangers, uncircumcised in heart and uncircumcised in flesh, to be in My sanctuary to pollute it…. They have broken My covenant!" What application does this have to baptized Baptists, who regard their own babies as strangers to God but yet bring them to worship Him?
In Luke 7:30, God declares that "the Pharisees and lawyers rejected the counsel of God against themselves, being not baptized." Commented Calvin: "It was already an evidence of their piety, that they [the godly] presented themselves to be baptized…. The scribes, in despising the baptism of John, shut against themselves, through their pride, the gate of faith…. Let us first guard against despising the very least of God's invitations, and be prepared in humility to commence with small and elementary instructions!"
In Acts 2:38f, God commands the penitent: "Be baptized every one of you…, for the promise is unto you and to your children!" Commented Calvin: "This passage therefore sufficiently refutes the Anabaptists, who deny baptism to the children of the faithful while they are still infants, as though they were not members of the Church…. This gross presumption is of no profit to them!" And on Acts 11:16f: "Those who are opposing infant baptism, are waging war against God!"
According to the Westminster Larger Catechism 124-130, the Fifth Commandment requires fathers and mothers not to commit "sins" by "the neglect of the duties required of them" — such as that of bringing their children to be baptized. "Second Kings 5:13; Ephesians 6:4; Deuteronomy 6:6f; Ezekiel 34:2-4."
Indeed, the Westminster Larger Catechism 166 requires that "infants descending from parents either both or but one of them professing faith in Christ…are…to be baptized. Genesis 17:7f; Galatians 3:9f; Colossians 2:11f; Acts 2:38f; Romans 4:11f; First Corinthians 7:14; Matthew 28:19; Luke 18:15f; Romans 11:16."
Rightly does the Westminster Confession 28:4f therefore conclude that "also the infants of one or both believing parents are to be baptized…. It be a great sin to contemn or neglect this ordinance! Luke 7:30 & Exodus 4:24-26."