A passage quoted by the heretics against repentance is explained in two ways, the first being that Hebrews 6:4 refers to the impossibility of being baptized again; the second, that what is impossible with man is possible with God.
6. Being then refuted by the clear example of the Apostle and by his writings, the heretics yet endeavour to resist further, and say that their opinion is supported by apostolic authority, bringing forward the passage in the Epistle to the Hebrews: “For it is impossible that those who were once enlightened, and have tasted the heavenly gift, and have been made partakers of the Holy Spirit, and have tasted the good word of God, and the powers of the world to come, should if they fall away be again renewed unto repentance, crucifying again the Son of God, and put Him to open shame.”
7. 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 which 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 secondly, it was right to set forth in a well-reasoned argument that baptism is not to be repeated.
8. And that the writer was speaking of baptism is evident from the very words in which it is stated that it is impossible to renew unto repentance those who were fallen, inasmuch as we are renewed by means of the laver of baptism, whereby we are born again, as Paul says himself: “For we are buried with Him through baptism into death, that, like as Christ rose from the dead through the glory of the Father, so we, too, should walk in newness of life.” And in another place: “Be renewed in the spirit of your mind, and put on the new man which is created after God.” And elsewhere again: “Your youth shall be renewed like the eagle,” because the eagle after death is born again from its ashes, as we being dead in sin are through the Sacrament of Baptism born again to God, and created anew. So, then, here as elsewhere, he teaches one baptism. “One faith,” he says, “one baptism.”
9. This, too, is plain, that in him who is baptized the Son of God is crucified, for our flesh could not do away sin unless it were crucified in Jesus Christ. And then it is written that: “All we who were baptized into Jesus Christ were baptized into His death.” And farther on: “If we have been planted in the likeness of His death, we shall be also in the likeness of His resurrection, knowing that our old man was fastened with Him to His cross.” And to the Colossians he says: “Buried with Him by baptism, wherein you also rose again with Him.” Which was written to the intent that we should believe that He is crucified in us, that our sins may be purged through Him, that He, Who alone can forgive sins, may nail to His cross the handwriting which was against us. In us He triumphs over principalities and powers, as it is written of Him: “He made a show of principalities and powers, triumphing over them in Himself.”
10. 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, who triumph, as it were, when we take to ourselves the likeness of His death, who 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.
11. But what of the passage wherein the doctrine of baptisms is spoken of? Because under the Law there were many baptisms or washings, he rightly rebukes those who forsake what is perfect and seek again the first principles of the word. He teaches us that the whole of the washings under the Law are done away with, and that there is one baptism in the sacraments of the Church. But he exhorts us that leaving the first principles of the word we should go on to perfection. “And this,” he says, “we will do, if God permits,” for no one can be perfect without the grace of God.
12. And indeed I might also say to any one who thought that this passage spoke of repentance, that things which are impossible with men are possible with God; and God is able whenever He 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 obtain. For it seemed impossible that water should wash away sin, and Naaman the Syrian thought that his leprosy could not be cleansed by water. But that which was impossible God made to be possible, Who gave us so great grace. In like manner 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 a true reasoning, he convinces us that the reiteration by any one of the Sacrament of Baptism is not permitted.
Source: On Repentance (New Advent)