Proverbs 30:18

An eagle and a ship and a serpent are not of one family and nature but of a distinguishable and different substance, and yet they are three. On the testimony of Scripture, therefore, [the Arians] learn that their arguments are against themselves.Therefore, in saying that the substance of the Father and of the Son is diverse and their Godhead distinguishable, [the Arians] themselves assert there are two gods. But we, when we confess the Father and the Son, in declaring them still to be of one Godhead, say that there are not two gods but one God. And this we establish by the Word of the Lord.

"There be three things which I cannot understand, and the fourth I know not: the tracks of an eagle flying," i.e., Christ's ascension; "and the ways of a serpent upon a rock," i.e., that the devil did not find a trace of sin in the body of Christ; "and the ways of a ship crossing the sea," i.e., the ways of the Church, which is in this life as in a sea, and which is directed by her hope in Christ through the cross; "and the ways of a man in youth," -the ways of Him, namely, who is born of the Holy Spirit and the Virgin. For behold, says the Scripture, a man whose name is the Rising.

Three things are difficult for me, etc. As far as history is concerned, it is as it is written; but because he speaks through parables, the way of an eagle in the sky is difficult to find: that is, the subtlety of the enemy, which flies around the hearts of heavenly men, is discovered with great labor. Similarly, the way of a serpent on the ground, that is, the cunning of the poisonous enemy, never ceasing to lay snares for those he sees founded on the rock of faith. Likewise, the way of a ship in the midst of the sea, that is, the way of iniquity, which runs through the bitter waves of this world, driven by the blasts of unclean spirits, with such subtlety that its path can hardly or not at all be discovered. For that the eagle signifies the malicious enemy, the prophet testifies, who says: Our persecutors were swifter than the eagles of the sky (Lamentations IV), that is, men who persecuted us were so cruel that they seemed to be more savage than the demons themselves. That the serpent often signifies the devil is proven both in the transgression of the first man and when a wise man said, Flee from sin as from the face of a serpent (Ecclesiasticus XXI). The ship, which signifies lightness, indicates those who are tossed about by every wind of doctrine, as the prophet demonstrates when he says, "And the Chaldeans in their ships glorifying" (Isa. XLIII); that is, the unclean spirits exalting themselves from the fragility and instability of the human mind, and rejoicing as victors over its ruin. Just as those things are so pure that they cannot be found, so is it difficult to comprehend how a youth who has deserted the way of truth can deviate with a wandering mind into countless thoughts every hour. Such is also the way of an adulterous woman, who eats and wipes her mouth and says, "I have done no wickedness." The adulterous mind is always unstable and wandering like those mentioned above. Whoever can conceal the vile act that they committed denies having done any crime.