The language "means of grace," refers to the idea that God uses mundane (ordinary) things (which we identify as Word, Sacrament, and Prayer) in order to communicate himself to his people for their good. These means--specifically these--have his promises attached to them (see Scripture), promises to bring blessings of salvation through a right use of them. The ordinary supplies extraordinarily by God's appropriation, when such is received by faith.
In Reformed theology, God's effectual work of salvation is specially directed to his elect people, the people for whom said benefits were procured. Those benefits are eternal, but also won in history by the work of Jesus Christ
extra nos (outside of us); as well as being applied personally to the elect in their own historic circumstances. It is in the receptive act of personal faith (also an element of God's gift of salvation), that God's work for man is particularized. Faith is
instrumental, it is
that by which Christ is apprehended, his mercies conveyed, and covenant-union instantiated.
Westminster Confession of Faith 28:6
The efficacy of Baptism is not tied to that moment of time wherein it is administered; yet notwithstanding, by the right use of this ordinance, the grace promised is not only offered, but really exhibited and conferred, by the Holy Ghost, to such (whether of age or infants) as that grace belongeth unto, according to the counsel of God’s own will, in His appointed time.
This paragraph conceptually distinguishes between subjective engagement in the event of a baptism, and subjective appreciation of the import of baptism. It expresses the theological understanding that these two items may (though not necessarily) be temporally separate with regard to a participant. It is expected that one's appreciation for baptism generally, and for his own baptism, will grow over time.
And thus, faith in the blessings that baptism proclaims succeeds in communicating those benefits to the believer not simply at one point (possibly at the event of his own baptism), but actually throughout his whole life of faith. Note that this explanation is sufficiently broad to cover any and all baptisms of God's elect. It is not tailored to an infant's baptism, nor to a more mature person's. It is simply the Reformed/Presbyterian doctrine of baptism. The proper subjects of baptism are determined not by an appeal to who--whether professors or their children--are definitely (or more likely to be) among the elect, for whom alone the promises of the covenant are meant for salvation. The prior question is: who are properly identified as those belonging to the visible church? Those, and only those persons are the proper subjects of baptism.
Hence, the principle of covenant-inclusion extended to the believer's children. In our view, in Gen.17 those children were put visibly into
the covenant of Grace (an overarching covenant-ideal for man's salvation, distinct from and following after the broken covenant of Works, and encompassing all the post-fall covenant-series as various historic manifestations of the divine salvation plan). And they have never since been visibly excluded; but rather the Abrahamic promises found in Gen.17 were reiterated in Act.2:28-39, at the beginning of the New Covenant era. Ergo, we conclude that the mark of divine proprietorship (which has replaced the mark of circumcision, Col.2:11-12) is rightly affixed to those childish persons who are themselves the proprietary constituents of their parent's estate, when even one of those parents regards himself as God's property (1Cor.7:14).
Finally (though there is far more to the doctrine of baptism than these few paragraphs), consider what
can be communicated to an infant child. To the active mind of a tiny infant, a thousand inputs every day are forming his first impressions of the world. It would be fair to say that sensory inputs have far greater effect at this time than verbal specifics. Yet those sensory inputs are conveying information to the nascent mind, contributing to its formation. Water upon the skin--it might seem too ordinary to convey anything of great value, let alone eternal value. But the Lord used mud and saliva to give sight to the blind man, Jn.9:6-7. Is he not capable of communicating something of his love to an infant, using mere water, if he so chose? If a mother can communicate something of her love to the child in caresses and murmurings and milk and smiles, cannot almighty God do the same?
If baptism is primarily me saying something to the world, or to God, then baptism is more of a "bare sign" than an avenue of communication. But the Reformed confess baptism is primarily God saying something to me, and to the world, "As surely as water washes away the dirt from the body, so certainly My blood and Spirit wash away the impurity of the soul, that is, all his sins. Even of this one being baptized, only let him believe this gospel. He who believes and is baptized will be saved, but he who does not believe will be condemned, Mk.16:16." See Heidelberg Catechism #69 & #71.
