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Just as it is hazardous to build a doctrine of baptism from Scripture simply by examining verses having to do with water, it is equally dangerous to try to build a robust view of baptism by simply looking at one chapter in the Confession.
Whenever union with Christ is present, so is saving faith (and visa versa). The WCF teaches that saving faith is “ordinarily” wrought by the ministry of the Word. The Confession most unambiguously steps out and discloses a view on God’s ordinary means of conferring the instrumental cause of justification, which is always accompanied by all the benefits of Christ’s work of redemption. There is no mention of the sacraments in this chapter on saving faith, other than teaching that the sacraments (along with prayer) strengthen, but do not produce, that which we receive by faith (not baptism!). Even more significant is that in its chapter on effectual calling, the Confession also indexes effectually calling not to baptism, but to Word and Spirit. In effectual calling, wrought by Word and Spirit and not baptism, the Confession teaches that God replaces the unbelieving heart of stone with a regenerate heart of flesh, the very work that many want to attribute to the rite of baptism. In a word, the Confession attributes that which baptism signs and seals not to the sign and seal of baptism but to the effectual working of Word and Spirit. The sacraments along with prayer serve to strengthen these realities (that are effected by other means than baptism).
At the very least, those with FV tendencies have irreconcilable differences with the Westminster standards. That is because they will not make conscience of the Confession’s teaching that sacraments in general and baptism in particular are “efficacious” in that they “confirm(!)” our interest in Christ, which we inherit through the effectual working of Word and Spirit, which together unite us to Christ. The chapter on the sacraments plainly teaches that baptism is a confirmatory seal and not a converting ordinance. Baptism confirms that which Word promises and Word and Spirit effect. The role of the sacraments are not intended to effect that which the Confession teaches is offered in Word and effected by Word and Spirit, but rather they are to effect the confirmation of what is effected by Word and Spirit. In other words, the Confession teaches that together Word and Spirit effect the reality (union with Christ), and the sacraments effect the confirmation of that effectuated reality.
All of that is not to say that conversion cannot be accompanied by baptism or that baptism cannot be given increase by the intelligible Word, resulting in Word-Spirit conversion. Notwithstanding, the Confession explicitly states that the gift of saving faith is ordinarily wrought through the administration of the Word (as opposed to baptism) and that the precursor to faith, effectual calling (wherein a sinner is recreated in Christ) comes not by baptism but by Word and Spirit. The place of baptism in particular is that by Word and Spirit it “confirms” that which is granted to us in our effectual calling etc. So, in sum, when we read in chapter 28 of the Confession about the efficacy of baptism, we must interpret “efficacy” according to chapter 27 on the sacraments, which states that the role of baptism is to confirm our interest in the offered promise, and not to effect what the promise contemplates. We must interpret Confession by conmparing it with Confession, no less than we are to interpret Scripture by Scripture.
Sacraments effect confirmation, plain and simple. They are not given to make effectual the reality of what is confirmed in the sacrament. Sacraments don’t create; they by grace sustain. Again though, baptism may certainly accompany the converting work of Word and Spirit, but it need not even do that in the life of the believer.