19 But which is the worse, not to be baptized at all, or to be twice baptized, it is difficult to decide. I see, indeed, which is more repugnant and abhorrent to men's feelings; but when I have recourse to that divine balance, in which the weight of things is determined, not by man's feelings, but by the authority of God, I find a statement by our Lord on either side. For He said to Peter, "He who is washed has no need of washing a second time;" and to Nicodemus, "Unless a man be born of water and of the Spirit, he cannot enter into the kingdom of God." What is the purport of the more secret determination of God, it is perhaps difficult for men like us to learn; but as far as the mere words are concerned, any one may see what a difference there is between "has no need of washing," and "cannot enter into the kingdom of heaven."
The Church, lastly, herself holds as her tradition, that without baptism she cannot admit a man to her altar at all; but since it is allowed that one who has been rebaptized may be admitted after penance, surely this plainly proves that his baptism is considered valid. If, therefore, Cyprian thought that those whom he considered to be unbaptized yet had some share in pardon, in virtue of the bond of unity, the Lord has power to be reconciled even to the rebaptized by means of the simple bond of unity and peace, and by this same compensating power of peace to mitigate His displeasure against those by whom they were rebaptized, and to pardon all the errors which they had committed while in error, on their offering the sacrifice of charity, which covers the multitude of sins; so that He looks not to the number of those who have been wounded by their separation, but to the greater number who have been delivered from bondage by their return.
For in the same bond of peace in which Cyprian conceived that, through the mercy of God, those whom he considered to have been admitted to the Church without baptism, were yet not severed from the gifts of the Church, we also believe that through the same mercy of God the rebaptized can earn their pardon at His hands.
Source: On Baptism, Against the Donatists (New Advent)