John 21:11, “So Simon Peter went over and dragged the net ashore full of one hundred fifty-three large fish.” --Many saints and exegetes have wondered; Why does St. John specify that exactly 153 fish were caught? What is the significance of this number?
Indeed, there is great diversity of opinion in this matter, but one thing that all agree on is this: the great catch of fish signifies that salvation is open to all and that the Church will encompass men from every nation, place, class, and time. The Fathers of the Church (and especially St. Augustine) were very interested in numbers, particularly in the various combinations of numbers which make up other numbers. In our consideration of the number 153 there are 5 core numbers to keep in mind: 100, 50, 10, 7, and 3. These numbers were used by the Fathers of the Church to explain the mystical meaning of this text.
St. Augustine says: The catch of fish tells us of the salvation of men, but man cannot be saved without keeping the 10 commandments. But, on account of the fall, man cannot even keep the commandments without the help of grace and the 7 gifts of the Holy Spirit. Moreover, the number 7 signifies holiness, since God blessed the 7th day and made it holy (Gen 2:3). But 10 plus 7 equals 17, and if all the numbers from 1 to 17 are added together (1+2+3…+17), they equal 153. Hence, the 153 fish signify that all the elect are to be saved by the gift of grace (7) and the following of the commandments (10).
Or rather: St. Augustine notes that there were 7 disciples in the boat (Peter, Thomas, Nathanael, the sons of Zebedee, James and John, and two other disciples), who had all been filled with the 7 gifts of the Holy Spirit. 7 times 7 equals 49. 49 plus 1 makes the perfection of 50. Now, 3 is the number of the Trinity and also of our faith (which is founded on the Trinity); but 50 times 3 (for our faith) is 150, plus 3 (for the Trinity) is 153. Hence, the 153 fish signify the fullness of the Church (7), filled with the Holy Spirit (7), perfected (50) in her faith (3) in the most holy Trinity (3).
St. Gregory the Great says: 10 and 7 are perfect numbers, which added together make 17. This, times 3, for the perfection of faith in the Trinity, makes 51. This, times 3 again, makes 153.
St. Cyril breaks 153 into 100 (the great number of gentiles to be saved), plus 50 (the smaller number of Jews to be saved), plus 3 (the Trinity who saves all). Others follow St. Cyril, but modify this as follows: 100 (the multitude of married lay faithful in the Church), plus 50 (the many faithful who commit themselves later in life to continence either living as widows or living with their spouse in a brother-sister relationship), plus 3 (the precious few who commit their whole lives to celibacy as virgins) equals 153 (the whole Church taken together as a single body).
Perhaps most convincing of all is the theory of St. Jerome: It was thought at that time that there were only 153 species of fish in all the world. Hence, the disciples caught 153 fish, signifying that men of every class and time would be saved through the Gospel.
For more, see the Catena Aurea on St. John of St. Thomas Aquinas and the Commentary on the Gospel of St. John of Cornelius a' Lapide.