Hello Andrew,
Here are two paragraphs from my book,
(Grandville: Reformed Fellowship, 2006), 26–27:
When we say that baptism is a sign of already-being-saved we forget what is so amazing about the story of Abraham. In Romans 4:11 we are told that Abraham’s circumcision was a seal of the righteousness that comes by faith. Abraham was a believer already. As Genesis 15:6 says, “He believed the LORD, and he counted it to him as righteousness.” So he was circumcised after he believed. But what did God then command Abraham to do in Genesis 17? He was commanded to administer the sign of circumcision to his seed. As the story unfolds, Abraham takes his son Ishmael, all those born in his household and household servants, and circumcises them. Later, after Isaac is born, Abraham circumcises him although he is only eight days old and has given absolutely no indication that he believed already. Abraham, then, was circumcising people before they professed faith in the LORD. Circumcision was still a sign of the righteousness that comes by faith, but since there is not a simultaneous relationship, the sign could be given first, then latter in Isaac’s life the blessing of the sign, righteousness by faith, could come. We, as Abraham’s children (Gal. 3:7), need to follow Abraham’s example. Although we may not understand how a baby can be baptized now and come to faith later, this is how God has worked throughout history. He did it then and he can do it now – he does do it now!
Thus God’s covenant sign of baptism is to be given to the children of believers because it has replaced circumcision. The same “seed” or descendants of Abraham that were circumcised in the Old Testament are the same “seed” that are now baptized. As we have seen above in the equating of circumcision and baptism: “The
essence of the sign was never abolished. The
form was.” This means that God’s grace is the same throughout the covenants, although the sign is different: circumcision then, baptism now.