7 Let no one, however, separate these distinguished apostles. In that which was signified by Peter, they were both alike; and in that which was signified by John, they will both be alike hereafter. In their representative character, the one was following, the other tarrying; but in their personal faith they were both of them enduring the present evils of the misery here, both of them expecting the future good things of the blessedness to come. And such is the case, not with them alone, but with the holy universal Church, the spouse of Christ, who has still to be rescued from the present trials, and to be preserved in the future happiness.
And these two states of life were symbolized by Peter and John, the one by the one, the other by the other; but in this life they both of them walked for a time by faith, and the other they shall both of them enjoy eternally by sight. For the whole body of the saints, therefore, inseparably belonging to the body of Christ, and for their safe pilotage through the present tempestuous life, did Peter, the first of the apostles, receive the keys of the kingdom of heaven for the binding and loosing of sins; and for the same congregation of saints, in reference to the perfect repose in the bosom of that mysterious life to come did the evangelist John recline on the breast of Christ.
For it is not the former alone but the whole Church, that binds and looses sins; nor did the latter alone drink at the fountain of the Lord's breast, to emit again in preaching, of the Word in the beginning, God with God, and those other sublime truths regarding the divinity of Christ, and the Trinity and Unity of the whole Godhead. which are to be yet beheld in that kingdom face to face, but meanwhile till the Lord's coming are only to be seen in a mirror and in a riddle; but the Lord has Himself diffused this very gospel through the whole world, that every one of His own may drink thereat according to his own individual capacity.
There are some who have entertained the idea— and those, too, who are no contemptible handlers of sacred eloquence— that the Apostle John was more loved by Christ on the ground that he never married a wife, and lived in perfect chastity from early boyhood. There is, indeed, no distinct evidence of this in the canonical Scriptures: nevertheless it is an idea that contributes not a little to the suitableness of the opinion expressed above, namely, that that life was signified by him, where there will be no marriage.
Source: Tractates on the Gospel of John (New Advent)