15 Then at length John descends to bear company with the other three, whose course is with the same Lord, but upon the earth, and joins them in recording the feeding of the five thousand men with the five loaves. In this narrative, however, he is the only one who mentions, that when the people wished to make Him a king, Jesus departed into a mountain Himself alone. And in making that statement, his intention appears to me to have been just to communicate to the reasonable soul the truth, that Christ reigns over our mind and reason purely in a sphere in which He is exalted above us, in which He has no community of nature with men, and in which He is verily by Himself alone, as He is the Father's only fellow.
This, however, is a mystical truth, which escapes the cognizance of carnal men, whose life creeps upon the lower soil of this earth, just because it is so sublime a mystery. Hence Christ Himself also departs into the mountain from the men whose habit is to seek for His kingdom with earthly conceptions of it. Thus is it that He expresses Himself elsewhere to this effect, “My kingdom is not of this world.” And this, again, is something which is reported only by John, who soars high over earth in a kind of ethereal flight, and delights himself in the light of the Sun of righteousness.
Then, on passing from the narrative connected with this mountain, and from the miracle of the five loaves, he still keeps company with the same three for a little while, until the notice of the crossing of the sea is reached, and the occasion on which Jesus walked upon the waters. But at this point he at once rises again to the region of the Lord's discourses, and relates those words, so grave, so lengthened, so sustainedly lofty and elevated, which had their occasion in the multiplying of the bread, when He addressed the multitudes to the following effect: “Verily, verily, I say unto you, you seek me, not because ye saw the miracles, but because ye did eat of the loaves, and were filled.
Labour not for the meat which perishes, but for that meat which endures unto everlasting life.” After which sayings, He continues to discourse in similar terms for a very long period, and in the most exalted strain. At that time, some fell away from the sublime teaching of such words, namely, those who walked no more with Him afterwards. But there were also those who did cleave to Him; and these were they who were able to receive the meaning of this saying, “It is the spirit that quickens, but the flesh profits nothing.” For surely it is true, that even through the flesh it is the spirit that profits, and the spirit alone that profits; whereas the flesh without the spirit profits nothing.
Source: The Harmony of the Gospels (New Advent)