19 But, my brethren, it is madness to say that— I will not say Judas— but that any man was better than he of whom it was said, that “Among those that are born of women, there has not arisen a greater than John the Baptist.” No servant then is preferred to him; but the baptism of the Lord, even when given through an evil servant, is preferred to the baptism even of a servant who was a friend. Listen to the sort of persons whom the Apostle Paul mentions, false brethren, preaching the word of God through envy, and what he says of them: “And I therein do rejoice, yea, and will rejoice.” They proclaimed Christ, through envy indeed, but still they proclaimed Christ.
Consider not the why, but the whom: through envy is Christ preached to you. Behold Christ, avoid envy. Do not imitate the evil preacher, but imitate the Good One who is preached to you. Christ then was preached by some out of envy. And what is envy? A shocking evil. By this evil was the devil cast down; this malignant pest it was which cast him down; and certain preachers of Christ were possessed by it, whom, nevertheless, the apostle permitted to preach. Wherefore? Because they preached Christ.
But he who envies, hates; and he who hates, what is said concerning him? Listen to the Apostle John: “He who hates his brother is a murderer.” Behold, after John baptism was given, after a murderer baptism was not given; because John gave his own baptism, the murderer gave the baptism of Christ. That sacrament is so sacred that not even the ministration of a murderer pollutes it.
Source: Tractates on the Gospel of John (New Advent)