8 Therefore this follows: “And of His fullness have all we received.” What have ye received? “And grace for grace.” For so run the words of the Gospel, as we find by a comparison of the Greek copies. He does not say, And of His fullness have all we received grace for grace; but thus He says: “And of His fullness have all we received, and grace for grace,”— that is, have we received; so that He would wish us to understand that we have received from His fullness something unexpressed, and something besides, grace for grace.
For we received of His fullness grace in the first instance; and again we received grace, grace for grace. What grace did we, in the first instance, receive? Faith: walking in faith, we walk in grace. How have we merited this? By what previous merits of ours? Let not each one flatter himself, but let him return into his own conscience, seek out the secret places of his own thoughts, recall the series of his deeds; let him not consider what he is if now he is something, but what he was that he might be something: he will find that he was not worthy of anything save punishment.
If, then, you were worthy of punishment, and He came not to punish sins, but to forgive sins, grace was given to you, and not reward rendered. Wherefore is it called grace? Because it is bestowed gratuitously. For you did not, by previous merits, purchase that which you received. This first grace, then, the sinner received, that his sins were forgiven. What did he deserve? Let him interrogate justice, he finds punishment; let him interrogate mercy, he finds grace. But God promised this also through the prophets; therefore, when He came to give what He had promised, He not only gave grace, but also truth. How was truth exhibited? Because that was done which had been promised.
Source: Tractates on the Gospel of John (New Advent)