Ephes. 2:11-13 (ESV)
Therefore remember that at one time you Gentiles in the flesh, called "the uncircumcision" by what is called the circumcision, which is made in the flesh by hands—  remember that you were at that time separated from Christ, alienated from the commonwealth of Israel and strangers to the covenants of promise, having no hope and without God in the world.  But now in Christ Jesus you who once were far off have been brought near by the blood of Christ.
Covenants of promise. O.K. by now you probably know I am going to dissect that word covenant. The word diatheke once again translated covenant, when it really means Testament. Sure a Testament is a type of covenant, but it is pretty specific. Paul says these are covenants of promise as it is translated in the English. A Testament is a covenant of promise, God promising us. Here we see that even in the Old Testament God operated in the same way. He gave the Israelites testaments. He mentions one here before he even gets around to talking about our separation from Christ. He talks about circumcision. Done with hands. He adds that to show how baptism replaces that. To show that now circumcision is dead and useless. But it used to be a promise, a testament. God promised salvation and sealed it with circumcision. So the Israelites circumcised their children at 8 days old so they could share in that promise, be sealed to it. They didn’t have to worry is this child old enough? Does this child know what he is signing up for. Is he old enough to pledge himself to God and take that seriously? None of that came into play. Circumcision wasn’t a covenant in the sense of a two sided contract. It was a Testament, it included the child in the promises of God. The child wasn’t promising anything at 8 days old. God was promising himself to the child. Every time the boy went to pee on a tree he would be reminded of that promise and the grace of God. That can be a lot of reminders during the day, and at night when you get older.
But now we have an even better testament, covenant of promise. We have the blood of Christ, the New Testament in his blood, by which we are brought near to God. We are baptized in it, a circumcision of the heart by the work of the Holy Spirit. And we return to it Sunday after Sunday when we partake of the Lord’s Supper. We drink it and are forgiven, brought near to God. What wonderful testaments.