Ambrose of Milan

Could Paul teach in opposition to his own act? He had at Corinth forgiven sin through penance; how could he himself speak against his own decision? Since, then, he could not destroy what he had built, we must assume that what he says was different from, but not contrary to, what had gone before. For what is contrary is opposed to itself; what is different has ordinarily another meaning. Things that are contrary are not such that one can support the other. Inasmuch, then, as the apostle spoke of remitting penance, he could not be silent as to those who thought that baptism was to be repeated. And it was right first of all to remove our anxiety and to let us know that, even after baptism, if any sinned, their sins could be forgiven them, lest a false belief in a reiterated baptism should lead astray those who were destitute of all hope of forgiveness. And second, it was right to set forth in a well-reasoned argument that baptism is not to be repeated.…So, then, that which he says in this epistle to the Hebrews, that it is impossible for those who have fallen to be "renewed unto repentance, crucifying again the Son of God and putting him to open shame," must be considered as having reference to baptism, wherein we crucify the Son of God in ourselves that the world may be by him crucified for us. We triumph, as it were, when we take to ourselves the likeness of his death. We put to open shame upon his cross principalities and powers and triumphed over them, that in the likeness of his death we, too, might triumph over the principalities whose yoke we throw off. But Christ was crucified once and died to sin once, and so there is but one, not several baptisms.…
And indeed I might also say to anyone who thought that this passage spoke of repentance, that things which are impossible from the human point of view are possible with God. God is able whenever God wills to forgive us our sins, even those which we think cannot be forgiven. And so it is possible for God to give us that which it seems to us impossible to attain. For it seemed impossible that water should wash away sin, and Naaman the Syrian thought that leprosy could not be cleansed by water. But that which was impossible, God who gave us such great grace made to be possible. Similarly it seemed impossible that sins should be forgiven through repentance, but Christ gave this power to his apostles, which has been transmitted to the priestly office. That, then, has become possible which was impossible. But by true reasoning, the apostle convinces us that the reiteration by anyone of the sacrament of baptism is not permitted.