In the Old Testament, infants were to be circumcised because they were members of the covenant between God and his people Israel. Circumcision was the sign of this covenant:
9 God also said to Abraham, “As for you, you and your offspring after you throughout their generations are to keep my covenant. 10 This is my covenant between me and you and your offspring after you, which you are to keep: Every one of your males must be circumcised. 11 You must circumcise the flesh of your foreskin to serve as a sign of the covenant between me and you. 12 Throughout your generations, every male among you is to be circumcised at eight days old—every male born in your household or purchased from any foreigner and not your offspring. –Gen. 17:9-12 CSB
Ultimately, this physical circumcision in the flesh pointed to the circumcision of Christ–the circumcision done without hands (Col. 2:11-12). The Apostle Paul says that this circumcision of Christ was done by the Holy Spirit (Rom. 2:29).
Physical circumcision was never an end in itself. It was always meant to point to Christ and the salvation he accomplished (Col. 2:11-15).
For example, those who were only circumcised in the flesh but not in the heart were told to circumcise their hearts (Deut. 10:16). As mentioned above, this circumcision of the heart could only be wrought through the Holy Spirit.
When infants were circumcised in the flesh, it was a reminder to their parents that the infants needed to be circumcised in the heart as well. “For a person is not a Jew who is one outwardly, and true circumcision is not something visible in the flesh” (Rom. 2:28). This was the message all along.
We will address infant baptism next…