4 Now let us see what He intended to signify in the case of that one whom He Himself, keeping the mystery of unity, as I said before, deigned to heal out of so many sick folk. He found in the number of this man's years the number, so to speak, of infirmity: “He was thirty and eight years in infirmity.” How this number refers more to weakness than to health must be somewhat more carefully expounded. I wish you to be attentive; the Lord will aid us, so that I may fitly speak, and that you may sufficiently hear.
The number forty is commended to our attention as one consecrated by a kind of perfection. This, I suppose, is well known to you, beloved. The Holy Scriptures very often testify to the fact. Fasting was consecrated by this number, as you are well aware. For Moses fasted forty days, and Elias as many; and our Lord and Saviour Jesus Christ did Himself fulfill this number of fasting. By Moses is signified the law; by Elias, the prophets; by the Lord, the gospel. It was for this reason that these three appeared on that mountain, where He showed Himself to His disciples in the brightness of His countenance and vesture.
For He appeared in the middle, between Moses and Elias, as the gospel had witness from the law and the prophets. Whether, therefore, in the law, or in the prophets, or in the gospel, the number forty is commended to our attention in the case of fasting. Now fasting, in its large and general sense, is to abstain from the iniquities and unlawful pleasures of the world, which is perfect fasting: “That, denying ungodliness and worldly lusts, we may live temperately, and righteously, and godly in this present world.”
What reward does the apostle join to this fast? He goes on to say: “Looking for that blessed hope, and the appearing of the glory of the blessed God, and our Saviour Jesus Christ.” In this world, then, we celebrate, as it were, the forty days' abstinence, when we live aright, and abstain from iniquities and from unlawful pleasures. But because this abstinence shall not be without reward, we look for “that blessed hope, and the revelation of the glory of the great God, and of our Saviour Jesus Christ.”
In that hope, when the reality of the hope shall have come to pass, we shall receive our wages, a penny (denarius). For the same is the wages given to the workers laboring in the vineyard, as I presume you remember; for we are not to repeat everything, as if to persons wholly ignorant and inexperienced. A denarius, then, which takes its name from the number ten, is given, and this joined with the forty makes up fifty; whence it is that before Easter we keep the Quadragesima with labor, but after Easter we keep the Quinquagesima with joy, as having received our wages. Now to this, as if to the wholesome labor of a good work, which belongs to the number forty, there is added the denarius of rest and happiness, that it may be made the number fifty.
Source: Tractates on the Gospel of John (New Advent)