Notes, Critical and Explanatory, on the Holy Scriptures
Luke 5
"This is that ship which, according to Matthew, is still tossed by the waves, and, according to Luke, is filled with fishes ; so that you perceive
" Fasting, whereby the flesh is subdued, and corporal luxury punished, is not rejected here, for this fast is pleasing to God. How could He forbid His disciples to fast, while He, the Lord Himself, fasted, and whilst He declared that the most wicked spirits cannot be cast out but by fasting and prayers V St. Ambrose.
" Although the others are commanded to let down their nets, to Peter alone it is said: 'Launch out into the deep :' that is, into the depth of mystery. For what is so deep as the riches of divine knowledge, to know the Son of God, and to profess belief in His Divine Generation ■?— To this depth of mystery the Church is led by Peter." St. Ambrose.
You arrive at the cited line; the whole work is yours to read around it. The text is set from the cited public-domain edition.