Theodoret of Cyrus: Rebaptism Impossible

It is out of the question, he is saying, for those who have approached all-holy baptism, shared in the grace of the divine Spirit and received the type of the eternal goods to make their approach again and be granted another baptism. This is no different, in fact, from fixing the Son of God to the cross again, and besmirching him again with dishonor already shown: as he in his own case endured the passion once, we too likewise ought to share the passion with him once. We are buried with him in baptism, and we rise with him; so it is not possible for us to enjoy the gift of baptism again: "Having risen from the dead, Christ dies no more," remember; "death no longer has dominion over him. The death he died, he died to sin, whereas the life he lives, he lives to God." Our former self was crucified with him in baptism by receiving the type of death. ... Now, the apostle said this to teach the believers from Jews not to think all-holy baptism is like the Jewish baptisms: they did not wash away sins, but cleansed the body of apparent defilement - hence they were applied many times and frequently. This baptism of ours, on the contrary, is one only, for the reason that it involves the type of the saving passion and resurrection, and prefigures for us the resurrection to come.

- Theodoret of Cyrus (around A.D. 393 to around A.D. 457), Commentary on Hebrews, Chapter 6, in Theodoret of Cyrus, Commentary on the Letters of St. Paul, Volume 2, p. 159 (2001), Robert C. Hill translator.