8 For if we determine on the number that should indicate the law, what else can it be but ten? For we have absolute certainty that the Decalogue of the law, that is, those ten well-known precepts, were first written by the finger of God on two tables of stone. But the law, when it is not aided by grace, makes transgressors, and is only in the letter, on account of which the apostle specially declared, “The letter kills, but the spirit gives life.” Let the spirit then be added to the letter, lest the letter kill him whom the spirit makes not alive, and let us work out the precepts of the law, not in our own strength, but by the grace of the Saviour.
But when grace is added to the law, that is, the spirit to the letter, there is, in a kind of way, added to ten the number of seven. For this number, namely seven, is testified by the documents of holy writ given us for perusal, to signify the Holy Spirit. For example, sanctity or sanctification properly pertains to the Holy Spirit, whence, as the Father is a spirit, and the Son a spirit, because God is a spirit, so the Father is holy and the Son holy, yet the Spirit of both is called peculiarly by the name of the Holy Spirit.
Where, then, was there the first distinct mention of sanctification in the law but on the seventh day? For God sanctified not the first day, when He made the light; nor the second, when He made the firmament; nor the third, when He separated the sea from the land, and the land brought forth grass and timber; nor the fourth, wherein the stars were created; nor the fifth, wherein were created the animals that live in the waters or fly in the air; nor the sixth, when the terrestrial living soul and man himself were created; but He sanctified the seventh day, wherein He rested from all His works. The Holy Spirit, therefore, is aptly represented by the septenary number.
The prophet Isaiah likewise says, “The Spirit of God shall rest on Him;” and thereafter calls our attention to that Spirit in His septenary work or grace, by saying, “The spirit of wisdom and understanding, the spirit of counsel and might, the spirit of knowledge and piety; and He shall be filled with the spirit of the fear of God.” And what of the Revelation? Are they not there called the seven Spirits of God, while there is only one and the same Spirit dividing to every one severally as He will? But the septenary operation of the one Spirit was so called by the Spirit Himself, whose own presence in the writer led to their being spoken of as the seven Spirits.
Accordingly, when to the number of ten, representing the law, we add the Holy Spirit as represented by seven, we have seventeen; and when this number is used for the adding together of every several number it contains, from 1 up to itself, the sum amounts to one hundred and fifty-three. For if you add 2 to 1, you have 3 of course; if to these you add 3 and 4, the whole makes 10; and then if you add all the numbers that follow up to 17, the whole amounts to the foresaid number; that is, if to 10, which you had reached by adding all together from 1 to 4, you add 5, you have 15; to these add 6, and the result is 21; then add 7, and you have 28; to this add 8, and 9, and 10, and you get 55; to this add 11 and 12, and 13, and you have 91; and to this again add 14, 15, and 16, and it comes to 136; and then add to this the remaining number of which we have been speaking, namely, 17, and it will make up the number of fishes.
But it is not on that account merely a hundred and fifty-three saints that are meant as hereafter to rise from the dead unto life eternal, but thousands of saints who have shared in the grace of the Spirit, by which grace harmony is established with the law of God, as with an adversary; so that through the life-giving Spirit the letter no longer kills, but what is commanded by the letter is fulfilled by the help of the Spirit, and if there is any deficiency it is pardoned. All therefore who are sharers in such grace are symbolized by this number, that is, are symbolically represented.
This number has, besides, three times over, the number of fifty, and three in addition, with reference to the mystery of the Trinity; while, again, the number of fifty is made up by multiplying 7 by 7, with the addition of 1, for 7 times 7 make 49. And the 1 is added to show that there is one who is expressed by seven on account of His sevenfold operation; and we know that it was on the fiftieth day after our Lord's ascension that the Holy Spirit was sent, for whom the disciples were commanded to wait according to the promise.
Source: Tractates on the Gospel of John (New Advent)