6 “And a great wind blowing, the sea rose.” Darkness was increasing, discernment was diminishing, iniquity was growing. “When, therefore, they had rowed about twenty-five or thirty furlongs.” Meanwhile they struggled onward, kept advancing; nor did those winds and storms, and waves and darkness effect either that the ship should not make way, or that it should break in pieces and founder; but amid all these evils it went on. For, notwithstanding iniquity abounds, and the love of many waxes cold, and the waves do swell, the darkness grows and the wind rages, yet the ship is moving forward; “for he that perseveres to the end, the same shall be saved.” Nor is that number of furlongs to be lightly regarded.
For it cannot really be that nothing is meant, when it is said that, “when they had rowed twenty-five or thirty furlongs, Jesus came to them.” It were enough to say, “twenty-five,” so likewise “thirty;” especially as it was an estimate, not an assertion of the narrator. Could the truth be anything endangered by a mere estimate, if he had said nearly thirty furlongs, or nearly twenty-five furlongs? But from twenty-five he made thirty. Let us examine the number twenty-five. Of what does it consist?
Of what is it made up? Of the quinary, or number five. That number five pertains to the law. The same are the five books of Moses, the same are those five porches containing the sick folk, the same are the five loaves feeding the five thousand men. Accordingly the number twenty-five signifies the law, because five by five— that is, five times five— make twenty-five, or the number five squared. But this law lacked perfection before the gospel came. Moreover, perfection is comprised in the number six.
Therefore in six days God finished, or perfected, the world, and the same five are multiplied by six, that the law may be completed by the gospel, that six times five become thirty. To them that fulfill the law, therefore, Jesus comes. And how does He come? Walking upon the waves, keeping all the swellings of the world under His feet, pressing down all its heights. Thus it goes on, so long as time endures, so long as the ages roll. Tribulations increase, calamities increase, sorrows increase, all these swell and mount up: Jesus passes on treading upon the waves.
Source: Tractates on the Gospel of John (New Advent)