Ante-Nicene Fathers Volume 5
Cyprian: Epistles LVI-LVII
In Epistle LVI, Cyprian writes again to Cornelius, the pastor of the Roman congregation and praises God for his [Cornelius'] faithfulness to the Gospel through persecution. In doing so, Cyprian thanks God that Cornelius has upheld the unity of the church--unlike the schismatic Novatian. Cyprian ends with the encouragement to continue in mutual fellowship and prayer, each pastor and each congregation supporting the other in faithfulness.
Epistle LVII is Cyprian's letter to Lucius, who had been exiled but not martyred. Cyprian tells Lucius that he is no less a Christian because he hasn't been executed for the faith. While this letter is fine enough, the big interesting point comes in paragraph 3, where in addition to learning that Cornelius has been martyred, we see a compressed picture of Cyprian's view of the church as a unified organic whole composed of a pastor ("bishop"), elders ("presbyters,") and congregation.