Another passage of St. John is considered. The necessity of keeping the commandments of God may be complied with by those who, having fallen, repent, as well as by those who have not fallen, as is shown in the case of David.
53. Let us consider another similar passage: “He that believes in the Son has eternal life, but he that believes not the Son shall not see life, but the wrath of God abides on him.” That which abides has certainly had a commencement, and that from some offense, viz., that first he not believe. When, then, any one believes, the wrath of God departs and life comes. To believe, then, in Christ is to gain life, for “he that believes in Him is not judged.”
54. But with reference to this passage they allege that he who believes in Christ ought to keep His sayings, and say that it is written in the Lord's own words: “I have come a light into this world, that whosoever believes in Me may not abide in darkness. And if any man hear My word and keep it, I judge him not.” He judges not, and do you judge? He says, “that whosoever believes in Me may not abide in darkness,” that is, that if he be in darkness he may not remain therein, but may amend his error, correct his fault, and keep My commandments, for I have said, “I will not the death of the wicked, but the correction.” I said above that he that believes in Me is not judged, and I keep to this: “For I am not come to judge the world, but that the world may be saved through Me.” I pardon willingly, I quickly forgive, “I will have mercy rather than sacrifice,” because by sacrifice the just is rendered more acceptable, by mercy the sinner is redeemed. “I come not to call the righteous but sinners.” Sacrifice was under the Law, in the Gospel is mercy. “The Law was given by Moses, grace by Me.”
55. And again further on He says: “He that despises Me, and receives not My words, has one that judges him.” Does he seem to you to have received Christ's words who has not corrected himself? Undoubtedly not. He, then, who corrects himself receives His word, for this is His word, that every one should turn back from sin. So, then, of necessity you must either reject this saying of His, or if you cannot deny it you must accept it.
56. It is also necessary that he who leaves off sinning must keep the commandments of God and renounce his sins. We ought not, then, to interpret this saying of him who has always kept the commandments, for if this had been His meaning He would have added the word always, but by not adding it He shows that He was speaking of him who has kept what he has heard, and what he heard has led him to correct his faults; he has then kept what he has heard.
57. But how hard it is to condemn to penance for life one who even afterwards keeps the commandments of the Lord, let Him teach us Himself Who has not refused forgiveness. Even to those who do not keep His commandments, as you read in the Psalm: “If they profane My statutes and keep not My commandments, I will visit their offenses with the rod and their sins with scourges, but My mercy will I not take from them.” So, then, He promises mercy to all.
58. Yet that we may not think that this mercy is without judgment, there is a distinction made between those who have paid continual obedience to God's commandments, and those who at some time, either by error or by compulsion, have fallen. And that you may not think that it is only our arguments which press you, consider the decision of Christ, Who said: “If the servant knew his Lord's will and did it not, he shall be beaten with many stripes, but if he knew it not, he shall be beaten with few stripes.” Each, then, if he believes, is received, for God “chastens every son whom He receives,” and him whom He chastens He does not give over unto death, for it is written: “The Lord has chastened me sore, but He has not given me over unto death.”
Source: On Repentance (New Advent)