In his Dialogue with Trypho the Jew, early Greek apologist Justin Martyr recounts his conversation with a Jewish man and his companions. Written in the mid second century, it is an important document for understanding early Christianity, not least of all because of the many examples of how some ancient Christians handled the Old Testament.
One point which Trypho raises several times against Justin Martyr is the fact that Christians say they follow the Old Testament, but are often uncircumcised. Justin responds by showing that many Old Testament figures themselves were not like Adam and Noah and even Abraham (similar to Paul’s argument in Romans 4) was uncircumcised when God declared him righteous. (Interestingly enough, Justin never responds, to my memory at least, that Christians do not need circumcision because they are baptised.) He also responds by saying that what is truly important is a circumcised heart, and cites several Old Testament prophets to establish this. Justin explains,
We who have received the second circumcision with stone knives are indeed happy. For your first circumcision was and still is administered by iron instruments, in keeping with your hardness of heart. But our circumcision, which is the second, for it was instituted after yours, circumcises us from idolatry and every other sin by means of sharp stones, namely, by the words uttered by the Apostles of Him who was the Cornerstone and the Stone not cut with human hands. Indeed, our hearts have been so circumcised from sin that we even rejoice as we die for the name of that noble Rock, whence gushes forth living water for the hearts of those who through Him love the Father of all, and who proffers the water of life to those desiring it.1
1Dialogue with Trypho 114.4 (trans. T. B. Falls, ed. M. Slusser; Washington, D.C.: Catholic University Press, 2003).