Romans 2:25 For circumcision indeed is of value if you obey the law, but if you break the law, your circumcision becomes uncircumcision. 26 So, if a man who is uncircumcised keeps the precepts of the law, will not his uncircumcision be regarded as circumcision? 27 Then he who is physically uncircumcised but keeps the law will condemn you who have the written code and circumcision but break the law. 28 For no one is a Jew who is merely one outwardly, nor is circumcision outward and physical. 29 But a Jew is one inwardly, and circumcision is a matter of the heart, by the Spirit, not by the letter. His praise is not from man but from God.
Look at the words “outwardly” and “inwardly” above. They are very important to understanding why the infant males of believers were circumcised in the old testament, and why that practice continued into the new testament with a new sign where the infant children of believers are baptized in the early church, and continues still to this day. Both of those “outwardly” applied sacraments were signs of God’s covenant commitment, and in the case of children it is coupled with His command to we who are their believing parents to teach them to know God and His ways.
In the case of the sign being applied to children, it is not the outwardly applied covenant sign of circumcision or baptism that regenerates them inwardly (as Roman Catholicism wrongly teaches), but that God’s model is “family”. We can see that believers do tend to raise children that wind up also believing, and we loving parents should raise our children to have faith in Christ. The outward sign of the covenant has always been applied to children and God’s plan is that we, who know Christ, raise them as members of the Christ confessing community of believers we are a part of, and we their parents are to teach them (we Reformed teach them the word understood through the confessions, and catechisms) the faith that is their birthright, because the promise to we who believe is to us and our children (see Acts 2:39). We do this in the hope and expectation that they will inwardly appropriate the faith.
One example we see later in Romans 9 is where Paul points out Jacob and Esau. Both had received the sign of the covenant outwardly, but later in life, inwardly only Jacob believed, Esau did not. What was the difference? We’re told it’s because one was elect and the other was not.
However, this applying of the outward sign was given to both when they were infants. This is the way God outwardly administrates the Covenant of Grace in both it’s old and new administrations. The outward sign is put on the parents who believe when they come to faith in Christ, and then also to their children, because this outwardly applied sign is also part of a family commitment by the parents to teach the faith. That is the mandate to us, even though in God’s secret election some of our children might not ever wind up inwardly believing, it is significant that we see Esau is said to have forsaken his birthright. It’s the reason in Acts 2:23 says that the promise is for us and our children, because the covenant model hasn’t changed, just the way it is administered has changed. Males are no longer a federal heads in the same way they were during the time of the old administration, and so both male and female children receive the outward sign which is also no longer a bloody sign of cutting off, because Christ was cut off in His sacrifice on the cross.
In the new administration God still takes this family of believers model very seriously, because He takes the teaching of our children the faith, among a community of believers hearing the word preached to them every Sunday, very seriously. We their parents should personally teach them the faith. Not just stick them in a school or have some pastor teach them. There is nothing wrong with others teaching them, but it’s especially important that we parents personally teach them as that is God’s model and mandate.
The model is not to just consider them hellions until they make a decision to answer an altar call someday, but to teach them and raise them expectant and hopeful that God’s promise to us is for our children as well. In this we see that Abraham is the model. Abraham was chosen to teach the faith to his children, and so are we: “For I have chosen him, that he may command his children and his household after him to keep the way of the Lord …” – Gen 18:19
This is the Reformed model because it’s the biblical model, it’s the way the sacrament of Baptism is to be done. — That this promise of God is also to our children, even though they haven’t believed yet, and it’s later where God causes them to inwardly trust in Christ. Our hope for them is that they don’t forsake their birthright like Esau did. Baptism is a sign of God’s commitment to us, and in the case of our children the expectation is that we would follow biblical model of parental discipleship to them. Because we are chosen to teach the children of our household about Christ and His gospel for them.