12 “Nevertheless, let us go unto him. Then said Thomas, who is called Didymus, unto his fellow disciples, Let us also go, that we may die with Him. Therefore Jesus came, and found that he had [lain] in the grave four days already.” Much might be said of the four days, according to the wont of the obscure passages of Scripture, which bear as many senses as there is diversity of those who understand them. Let us express also our opinion of what is meant by one four days dead.
For as in the former case of the blind man we understand in a way the human race, so in the case of this dead man many perhaps are also to be understood; for one thing may be signified by different figures. When a man is born, he is born already in a state of death; for he inherits sin from Adam. Hence the apostle says: “By one man sin entered into the world, and death by sin; and so that passed upon all men, wherein all have sinned.” Here you have one day of death because man inherits it from the seed stock of death.
Thereafter he grows, and begins to approach the years of reason that he may know the law of nature, which every one has had implanted in his heart: What you would not have done to yourself, do not to another. Is this learned from the pages of a book, and not in a measure legible in our very nature? Have you any desire to be robbed? Certainly not. See here, then, the law in your heart: What you are unwilling to suffer, be unwilling to do. This law also is transgressed by men; and here, then, we have the second day of death.
The law was also divinely given through Moses, the servant of God; and therein it is said, “You shall not kill; you shall not commit adultery; you shall not bear false witness; honor your father and mother; you shall not covet your neighbor's property; you shall not covet your neighbor's wife.” Here you have the written law, and it also is despised: this is the third day of death. What remains? The gospel also comes, the kingdom of heaven is preached, Christ is everywhere published; He threatens hell, He promises eternal life; and that also is despised. Men transgress the gospel; and this is the fourth day of death. Now he deservedly stinks. But is mercy to be denied to such? God forbid; for to raise such also from the dead, the Lord thinks it not unfitting to come.
Source: Tractates on the Gospel of John (New Advent)