I looked at the proof-texts from the catechism, but I still don't see how that is translated into "we must baptize our infants." And I understand the logic of the Covenants, and the sign, but I still don't necessarily see the scriptural support that infants should be baptized. But, I will definitely study the issue and take a look at that book and the post you cited.
"The inclusion of children in the covenant is commanded, and a promise is attached to it."
How is this inclusion commanded through the means of infant baptism specifically?
Here is a quote from PCA pastor Wes White's
The Question of Infant Baptism.
The Question of Infant Baptism
Introduction – what is the significance of the promise?
The foundation of infant circumcision in the Old Testament was the statement, “I will be a God to you and to your descendants after you in their generations, for an everlasting covenant, to be God and to your descendants after you” (Gen. 17:7, see vv. 10 & 14). What is actually being promised here? Before we answer this question, it is helpful for us to understand what is not being said.
1. God was not promising to be the God of all the children of Abraham in an external sense. He was not saying that those children would simply profess the true religion, give outward obedience and worship, and publicly acknowledge God as their God. There is no hint in Scripture that this is what it means to have God as our God.
2. Consequently, it does not mean that every single child will be part of an external covenant. It does not mean that they will simply receive the offers of the covenant, “If you repent and believe, then you will be saved.” This may be a part of what it means to have God as our God, but it is not complete.
3. It does not mean that every child of Abraham head for head would have God as their God. This is plain from Romans 9:1-13 and from experience.
4. It did not mean that every child would get to have God as their God unless and until they broke the covenant or turned away from Abraham’s faith. It did not mean that all Abraham’s descendants would be saved by circumcision and that later some would fall away.
And so what did it mean?
“The descendants” referred to some of Abraham’s children or Abraham’s children in general. It meant that God would truly be the God of some of Abraham’s physical descendants. In other words, God would raise up a spiritual seed for Abraham from his physical descendants.
That “God would be their God” meant that they would have God in the fullest sense. God described this to Abraham in Gen. 15:1, “I am your shield and very great reward.” To have God as our reward means to be able to enjoy God forever. It includes being accounted righteous before God (Gen. 15:6). This means that they would not only obtain salvation but be kept in that salvation and finally brought to glory.
Now some may say, “Does having God as our God really mean all that?” According to Jesus it does. Jesus inferred from the phrase “I am the God of Abraham, Isaac, and Jacob” that there is a resurrection from the dead (see Mt. 22:31-33).
This principle continues on through the generations. As Moses spoke to the Israelites, he noted that even their sin would not cause this principle to cease, “And the LORD your God will circumcise your heart and the heart of your descendants, to love the LORD your God with all your heart and with all your soul, that you may live” (Deut. 30:6).
God promised nothing less than full salvation to Abraham and his physical descendants. This did not mean all of them, for Ishmael, Esau, and many in the wilderness were destroyed. Nevertheless, the Lord brought about not only a physical seed for Abraham but a spiritual seed for Abraham from that physical seed.
I believe the rest of this short article answers your question. White discusses the relevance of many of the Scriptures cited as proof-texts in the Heidelberg Catechism quote I put in my previous post.
I would personally answer your question by pointing to the meaning of circumcision and baptism. The sign of circumcision, according to the Apostle Paul, was a "seal of the righteousness
of the faith" (Rom. 4:11-12). What is interesting about this is that circumcision was administered to both believing Abraham, and to his infant Isaac
who could not profess faith (Gen. 17:26-27, 21:4). In fact, God explicitly commanded that the sign be administered to infants promising to be "be a God unto thee [Abraham], and
to thy seed after thee" (Gen. 17:9-14). Paul elsewhere tells us that those who believe in Christ whether Jew or Gentile are Abraham's seed, heirs according to the promise (Gal. 3:16, 29). The New Covenant is the fulfillment of the Abrahamic Covenant! On the day of Pentecost, Peter tells his hearers to repent and
be baptized for the "the promise [of the Spirit] is unto you, and
to your children, and to all that are afar off, even as many as the LORD our God shall call" (Acts 2:38-39; cf. 16:31). The promise of the Spirit is the same promise given to Abraham (Gal. 3:8, 14). Doesn't this strongly indicate that God is still a God of believers and their seed in the New Covenant?
The meaning of baptism is essentially the same as that of circumcision. This is explicitly seen in Col 2:11-12 which says that "In Him [Christ] also
you were circumcised with a circumcision made without hands...
having been buried with him in baptism.... (ESV)" We were circumcised when we were baptized! (This language is what the Westminster Confession calls "sacramental union" [WCF Chapter 27:2].) What did circumcision signify? Well, circumcision signified the same reality. Deut 30:6 says, "And the LORD thy God will
circumcise thine heart, and the heart of
thy seed..." (cf. Rom. 2:28-29). What is clear from these texts is that though the
sign changed (from the cutting off of the foreskin of the flesh to the washing with water in the Name of the Trinity), the
thing signified did not change.
Both circumcision and baptism signify the work of the Holy Spirit in regeneration, the
circumcision of the heart. If circumcision was administered to believers and their children, baptism should also be administered to believers and their children.
-----Added 7/23/2009 at 12:47:36 EST-----
The Westminster Confession, therefore, rightly affirms,
Chapter 28. Of Baptism.
4. Not only those that do actually profess faith in and obedience unto Christ,a but also the infants of one or both believing parents are to be baptized.b
a. Mark 16:15-16; Acts 8:37-38. • b. Gen 17:7, 9 with Gal 3:9, 14 and Col 2:11-12 and Acts 2:38-39 and Rom 4:11-12; Mat 28:19; Mark 10:13-16; Luke 18:15; 1 Cor 7:14.