This is a great question, but unfortunately, my answer can do nothing to sway your relative’s choice to get “re-baptized” seeing they already went through it. But this issue really gets to the heart of how Christians see baptism. Within the pale of “orthodox Christianity” you have two major views on baptism: (1) Those who see baptism as a “sign & seal” of the new covenant, and (2) those who see baptism as a “witness & testimony” of their faith in Jesus Christ. The first view is that held by confessional Reformed and Presbyterians. The second view is that held by conservative Baptists or Evangelicals who are “baptistic” in their theology of the sacraments.

Our confessional standards in the RCUS teach that baptism (along with the Lord’s Supper) is a “sign and seal” that “confirms” our faith in Christ and that we are partakers of all His benefits (Heidelberg Catechism LD25 Q65). Baptism is a “sign and seal” that points to the washing of our sins by the precious blood of Christ. That just as dirt is washed away by water, so too our sins are washed away by the blood of Christ (Heidelberg Catechism LD26 Q69). In Heidelberg Catechism LD27 Q74 the question is asked “are infants also to be baptized?” The Heidelberg answers:

Yes, for since they, as well as their parents, belong to the covenant and people of God, and through the blood of Christ both redemption from sin and the Holy Spirit, who works faith, are promised to them no less than to their parents, they are also by Baptism, as a sign of the covenant, to be engrafted into the Christian Church, and distinguished from the children of unbelievers, as was done in the Old Testament by circumcision, in place of which in the New Testament Baptism is appointed.

So we see that the authors of the Heidelberg Catechism saw baptism as not only a “sign and seal” of the washing away of sins by the blood of Christ, but also as a sign of the covenant that marked off the children of believers from the children of unbelievers just as circumcision served that purpose for the people of God in the OT (the children of Israel).

Now our Baptist brethren merely see baptism as testimony of one’s faith in Christ. The Southern Baptist Convention, the largest Baptist “denomination” in the US, says about baptism in the Baptist Faith and Message the following:

Christian baptism is the immersion of a believer in water in the name of the Father, the Son, and the Holy Spirit. It is an act of obedience symbolizing the believer’s faith in a crucified, buried, and risen Saviour, the believer’s death to sin, the burial of the old life, and the resurrection to walk in newness of life in Christ Jesus. It is a testimony to his faith in the final resurrection of the dead. Being a church ordinance, it is prerequisite to the privileges of church membership and to the Lord’s Supper.

Now I contend that our Baptist brothers and sisters not only have a deficient view of baptism, but also a deficient view of the covenant and the continuity of the people of God (the church being an expansion of the people of Israel). They will argue that you never see infant baptism in the NT. While technically that’s true, there are two things to say about that. First, an argument from silence is not necessarily proof. Consider this: The nation of Israel circumcised their children as a matter of law (Leviticus 12:3). If the NT church is really an expansion of Israel and baptism replaces circumcision as the sing of the covenant, then you really wouldn’t need a specific command to baptize your children. The command to baptize disciples would include their children.

This leads me to the second thing, in the Book of Acts, the promise that is signified in baptism is made to believers and their children. That’s what Peter says at the end of his Pentecost sermon (Acts 2:38-39). When Lydia is converted by Paul in Philippi, she is baptized with her household (Acts 16:15). If the promise that is sealed in baptism is for “you and your children” and we see households (which presumably had children) baptized, then we have Biblical warrant for the baptism of children.

Moreover, the Baptist view of baptism means the children of believers are no different than the children of unbelievers. Now practically speaking, Baptists parents are just as diligent in raising their children in the “nurture and admonition of the Lord” as Reformed parents are. They live in a happy inconsistency in that they treat their children as “covenant children,” while their theology says their children are not members of the covenant.

Finally, as it pertains to “re-baptism,” it should be obvious that one can (or should) no more be “re-baptized” as one could be “re-circumcised.” The Belgic Confession in article 34 (on “Holy Baptism”) says in part:

We believe, therefore, that every man who is earnestly studious of obtaining life eternal ought to be baptized but once with this only baptism, without ever repeating the same, since we cannot be born twice...Therefore we detest the error of the Anabaptists, who are not content with the one only baptism they have once received, and moreover condemn the baptism of the infants of believers, who we believe ought to be baptized and sealed with the sign of the covenant, as the children in Israel formerly were circumcised upon the same promises which are made unto our children.

I believe the Bible is very clear that there is one people of God (Israel in the OT, and the church in the NT). I also believe that the sign of the covenant was circumcision in the OT and is now baptism in the NT. Finally I believe that the sign of the covenant should be applied to all adult converts and their children.

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This same member of the congregation also asks: “I thought once you do your marriage vows in front of God, it is your confession?”

I agree that once you make your wedding vows in front of God and our friends and family, that should be it. Jesus tells us to “let your ‘Yes’ be ‘Yes.’ And your ‘No,’ ‘No’” (Matthew 5:37). Once we make a promise (an oath or a vow), no matter what the circumstances are, we should be bound to it.

Having said that, I do not feel as strongly about the renewal of wedding vows as I do about the re-baptism question. The context of Jesus’ comment in Matthew 5:37 is regarding people who attempt to circumvent their oaths by swearing falsely. In other words, they would swear by all sorts of things (the temple, the earth, the altar, etc.) and not in God’s name. Then when they would break their oath, they would say “well I didn’t swear in God’s name.”