All analogies ultimately fall short in one aspect or another but their main point remains.
In this case, the main analogy between circumcision and baptism is that they are both means of entering into a covenant with God. However, they are two different covenants so they have two different sets of circumstances. After all, even those of us Christians who teach infant baptism do not demand it be done on the 8th day *exactly *as circumcision was. This is because the new covenant frees us from the particulars of the laws of the Mosaic Law (Galatians 3:27-28), which would have limited the sign of the covenant to males.
