Revelation 16:13

In addition to their ugly appearance, frogs are unclean also in their places of habitation. Although they seem to be native to the waters, they cannot endure it when the waters recede or dry up, and they roll around in the waters themselves and in the mud and filth of the waters. The hypocrites likewise do not spend their time in the waters, as they seem to, but in filthy acts that those who believe lay aside in the water. Similar to frogs are such persons who are not ashamed to wallow in sins and vices, which others put aside through penance or baptism. Whenever someone converts to God and repents that he had been arrogant, adulterous, drunken or lustful, such a person imitates these sins that another has relinquished by confessing and thinking to himself says, “I do what I want and later I will do penance as that one has done penance.” But suddenly the last day overtakes him, and any confession is lost and his damnation remains. Such is that person who wishes to imitate others, not in that which is good but in that which is evil, and like frogs cover themselves and roll around in that muck and mire from which others have been liberated. Frogs, therefore, signify the spirits of demons who do signs and wonders.

In view of the various passions in all of the vexations and vices they cause, evil spirits are said to possess a diversity of characteristics. And so, rightly did [Jesus] say somewhere in the Gospel, “deaf and dumb spirit,” and elsewhere, “unclean spirit,” and elsewhere the prophet said, “a spirit of harlotry had led them astray,” for no other reason than that those under its control perform the uncleanliness of its own vice. Here, however, he includes all three unclean spirits, because he indicates that the one body of the devil is made up of its habitual differences, mentioning those [spirits] that possess the greater power, starting with the devil as the dragon; then the beast, which is the antichrist with his followers; and the false prophet, who represents the leaders and teachers of noxious doctrines. Therefore, he sees one spirit but mentions three for the number of the parts of the one body, so that every one of the lost is shown to be led by one malign spirit. Perhaps for this reason he adds the horrible filthiness of frogs as a comparison with their impudence. For like frogs living in the sewers at night, these false prophets make a great noise through the damnable ranting of their error. Just as frogs are loathsome in the places they inhabit, in their appearance and in their annoying croaking, so the devil with his followers is recognized to be abominable to the truth and is deservedly and justly damned to eternal fire. The blessed apostle says something similar when he says, “Do not be mismated with unbelievers. For what partnership have righteousness and iniquity? Or what fellowship has light with darkness? What accord has Christ with Belial? Or what has a believer in common with an unbeliever? What agreement has the temple of God with idols?” And so here, when it says that from their mouths came forth unclean spirits similar to frogs, it is as though it said, why do you compare filth to heaven when it is fitting as a habitation of frogs even as it is of the horrible blasphemies of the false prophets? He is claiming that the teachers of the truth are as far removed [from such persons] as frogs are from holy persons, so that those who are not able to recognize that their frauds must be refuted by the power of discretion, may at least, when confronted by the ugliness of frogs, attempt to avoid such persons who, like frogs, are a hindrance to the ignorant lest they receive the truth in the quiet practice of devotion.

This passage mentions individually the devil as a dragon, the antichrist as a beast, and the false prophet as another figure distinct from the other two. It says that spirits similar to frogs come out from these, for they are characterized by a poisonous and filthy nature, and these evil powers love the mud and creep toward foul and moist pleasures. By the commands of the devil and of the antichrist and of the false prophet, they will use their mouths to show false signs and wonders to humankind.

And I saw from the mouth of the dragon and from the mouth of the beast, etc. The spirit of the devil, and the Antichrist, and the leaders of his body, who are called threefold according to the number of parts of one body, are likened to frogs, which are horrible in place, appearance, and troublesome noise. Though they seem to be inhabitants of the waters, they wallow in filth and mud. Hypocrites, promising the water of life, hide dead in the filth which believers lay down in the water. Thus Pharaoh, who, in order to destroy the people in baptism, dared to enter after them, was killed there.

And he says they came out from the mouth of the Dragon and from the mouth of the False Prophet. The Dragon, the author of evil, he calls the Devil, and the False Prophet is someone else acting by the power of the Devil, as if it were prophesying, or the Antichrist.

But if he is speaking in general of the Antichrist, whom in the earlier passages he introduced as one deprived by the Spirit of the Lord, but now as though still living he vents demons through his mouth, let the hearer not be astonished.

For what is seen is a vision, and the first things are often shown to the evangelist later, and conversely the later things first, and he says, I saw three spirits like frogs. For there are spirits of demons performing signs, which go forth unto the kings. The demons are compared to frogs because they delight in the filthy, muddy life of men and in the slime. But the damp and windless life seems to them more like the life of sinners rather than the steadfast and hardened life of the righteous, whom they greatly envy and rejoice in the destruction of the living.