Justin Martyr's Dialogue with Trypho the Jew, Chapter 43.
And we, who have approached God through Him (Christ), have received not carnal, but spiritual circumcision, which Enoch and those like him observed. And we have received it through baptism, since we were sinners, by God's mercy; and all men may equally obtain it.
Justin lived from approximately 110 to 165. He was one of the earliest of the Early Church Fathers (which is why this passage appears in Vol. 1 of the Ante-Nicene Fathers, p. 216). This demonstrates that the identification of circumcision with baptism is not a later development of Reformed theology, in justification of infant baptism, but dates to the earliest annals of church history and Christian theology.