The question we all ask ourselves at one point or another is, why circumcision?
First, in the context of what God is speaking to Abraham, the covenant is dealing specifically with fathering nations. Circumcision dealt with the organ of reproduction, and that was no coincidence. But further than that, circumcision would have practical benefits.
Medical research indicates that circumcision decreases UTI and STI risk. It protects against some types of cancer and inflammation as well. Historically, Jewish women have an incredibly low rate of cervical cancer and doctors believe it is due to their husbands being circumcised.
Circumcision is literally the cutting away of the flesh… something Paul wrote about from a spiritual point of view.
For the flesh desires what is against the Spirit, and the Spirit desires what is against the flesh; these are opposed to each other, so that you don’t do what you want.
Galatians 5:17 HCSB
God was setting apart His people with a covenant that involved the cutting away of the flesh. Much later, Jesus would come to destroy sin and death and send to us the Holy Spirit who would battle the flesh.
“We have two natures—the old and the new. That is what Paul describes in the last part of Romans. He himself experienced the turmoil of two natures, and this has also been the experience of many believers. The flesh wars against the Spirit, and the Spirit wars against the flesh. Therefore, we cannot do the things that we would like to do. The new nature rebels against the old nature. They are contrary; they are at war with each other. Have you experienced this in your own life?”
J. Vernon McGee