Book 4 of 4 - External Means by which God Invites Us into the Society of Christ and Holds Us Therein
Chapter 14 - The Sacraments
21-26 - Relating OT sacraments to NT sacraments
21. Circumcision showed corrupt nature "needs pruning." It showed the righteousness of faith - Rom 4:11. Baptisms (purifications) showed our need for cleansing - by Christ - Heb 9:10, 14; 1 John 1:7; Rev 1:5. Sacrifices showed some satisfaction for sin needed, and a mediator/priest - Heb 4:14; 5:5; 9:11; Phil 2:8; Rom 5:19.
22. NT sacraments show Christ more clearly: baptism shows we are washed, the Supper shows we are redeemed. The Spirit is the source of these: Spirit, water, and blood: 1 John 5:8. The blood and water from Christ's side, Augustine called "the wellspring of our sacraments."
23. The Jews received not empty signs in their sacraments, but Christ - 1 Cor 10:3. They received Christ, too, just not as fully expressed.
24. Just because circumcision was "of the letter" (Rom 2:29) doesn't demean it below baptism, for baptism can be merely outward, too - 1 Pet 3:21. Paul argues against it as necessary to salvation (Col 2:11-12), not a legitimate sign. "Baptism is today for Christians what circumcision was for the ancients."
25. Paul and Hebrews appear to disparage the OT ceremonies because they were fulfilled in Christ. They were battling those seeking to go back to those ceremonies, to those distorting and misusing them superstitiously. OT rituals were right, when "directed to Christ."
26. Augustine overstated the difference a bit. "Faith remains while signs change." Receiving the sacraments is not a work of merit, as the scholastics said.