He must increase. John the Baptist proceeds farther; for, having formerly been raised by the Lord to the highest dignity, he shows that this was only for a time, but now that the Sun of Righteousness, (Malachi 4:2) has arisen, he must give way; and, therefore, he not only scatters and drives away the empty fumes of honor which had been rashly and ignorantly heaped upon him by men, but also is exceedingly careful that the true and lawful honor which the Lord had bestowed on him may not obscure the glory of Christ. Accordingly, he tells us that the reason why he had been hitherto accounted a great Prophet was, that for a time only he was placed in so lofty a station, until Christ came, to whom he must surrender his office. In the meantime, he declares that he will most willingly endure to be reduced to nothing, provided that Christ occupy and fill the whole world with his rays; and this zeal of John all pastors of the Church ought to imitate by stooping with the head and shoulders to elevate Christ.

He must increase - his authority and influence among the people must grow. his doctrine shall continue to spread until it extends through all the earth.
I must decrease - "The purpose of my ministry is to point men to him. When that is done my work is done. I came not to form a party of my own, nor to set up a religion of my own; and my teaching must cease when he is fully established, as the light of the morning star fades away and is lost in the beams of the rising sun. This evinced John's humility and willingness to be esteemed as nothing if he could honor Christ. It shows us, also, that it is sufficient honor for man if he may be permitted to point sinners to the Lord Jesus Christ. No work is so honorable and joyful as the ministry of the gospel; none are so highly honored as those who are permitted to stand near the Son of God, lead perishing men to his cross. Compare Daniel 12:3.

He must increase - His present success is but the beginning of a most glorious and universal spread of righteousness, peace, truth, and good will among men.
I must decrease - My baptism and teaching, as pointing out the coming Messiah, must cease; because the Messiah is now come, and has entered publicly on the work of his glorious ministry.

He must increase,.... Not in stature of body, or in wisdom and understanding of mind, as man, he being come to maturity in these things already; but in fame, credit, and reputation among men; as he afterwards did in the land of Judea, by reason of his miracles and doctrines; and after that among the Gentiles, through the publication of his Gospel; and will more and more in the latter day, when he, and he alone, shall be exalted: and he must increase in the ministry of his word, which was published by him, and his disciples, throughout all the cities of Israel; and which, after his resurrection and ascension, grew and increased mightily, notwithstanding the opposition made unto it both by their civil and ecclesiastical rulers; and which, by the means of his apostles, was spread throughout the Gentile world, and will hereafter cover the earth, as the waters do the sea: and also in his kingdom and interest, which at first were very small, like a grain of mustard seed, or like a little stone cut out of the mountain without hands; but in process of time grew exceedingly, and will, ere long, till the face of the whole earth; for the kingdoms of this world will become the kingdoms of our Lord, and of his Christ; and his dominion shall be from sea to sea, and from the river to the ends of the earth; and of the increase of it there shall be no end. And so likewise in the number of his followers, which at first were but few in Judea, but afterwards greatly increased, and especially among the Gentiles; and will be very numerous in the latter day glory, when the nation of the Jews will be born at once, and the fulness and forces of the Gentiles are brought in:
but I must decrease; as he did in his esteem among the people; see John 5:3; and in his work and office, which were now come to an end, Christ, whose forerunner he was, being come; and quickly after this he was put into prison, and there put to death.

He must increase, but I must decrease - So they who are now, like John, burning and shining lights, must (if not suddenly eclipsed) like him gradually decrease, while others are increasing about them; as they in their turns grew up, amidst the decays of the former generation. Let us know how to set, as well as how to rise; and let it comfort our declining days to trace, in those who are likely to succeed us in our work, the openings of yet greater usefulness.