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1 And as they went on their journey, they came in the evening to the river Tigris, and they lodged there. 2 And when the young man went down to wash himself, a fish leaped out of the river, and would have devoured him. 3 Then the angel said unto him, Take the fish. And the young man laid hold of the fish, and drew it to land. 4 To whom the angel said, Open the fish, and take the heart and the liver and the gall, and put them up safely. 5 So the young man did as the angel commanded him; and when they had roasted the fish, they did eat it: then they both went on their way, till they drew near to Ecbatane. 6 Then the young man said to the angel, Brother Azarias, to what use is the heart and the liver and the gal of the fish? 7 And he said unto him, Touching the heart and the liver, if a devil or an evil spirit trouble any, we must make a smoke thereof before the man or the woman, and the party shall be no more vexed. 8 As for the gall, it is good to anoint a man that hath whiteness in his eyes, and he shall be healed. 9 And when they were come near to Rages, 10 The angel said to the young man, Brother, to day we shall lodge with Raguel, who is thy cousin; he also hath one only daughter, named Sara; I will speak for her, that she may be given thee for a wife. 11 For to thee doth the right of her appertain, seeing thou only art of her kindred. 12 And the maid is fair and wise: now therefore hear me, and I will speak to her father; and when we return from Rages we will celebrate the marriage: for I know that Raguel cannot marry her to another according to the law of Moses, but he shall be guilty of death, because the right of inheritance doth rather appertain to thee than to any other. 13 Then the young man answered the angel, I have heard, brother Azarias that this maid hath been given to seven men, who all died in the marriage chamber. 14 And now I am the only son of my father, and I am afraid, lest if I go in unto her, I die, as the other before: for a wicked spirit loveth her, which hurteth no body, but those which come unto her; wherefore I also fear lest I die, and bring my father's and my mother's life because of me to the grave with sorrow: for they have no other son to bury them. 15 Then the angel said unto him, Dost thou not remember the precepts which thy father gave thee, that thou shouldest marry a wife of thine own kindred? wherefore hear me, O my brother; for she shall be given thee to wife; and make thou no reckoning of the evil spirit; for this same night shall she be given thee in marriage. 16 And when thou shalt come into the marriage chamber, thou shalt take the ashes of perfume, and shalt lay upon them some of the heart and liver of the fish, and shalt make a smoke with it: 17 And the devil shall smell it, and flee away, and never come again any more: but when thou shalt come to her, rise up both of you, and pray to God which is merciful, who will have pity on you, and save you: fear not, for she is appointed unto thee from the beginning; and thou shalt preserve her, and she shall go with thee. Moreover I suppose that she shall bear thee children. Now when Tobias had heard these things, he loved her, and his heart was effectually joined to her.
[AD 397] Ambrose of Milan on Tobit 6:1
What shall I say about dogs, who have a natural instinct to show gratitude and to serve as watchful guardians of their masters’ safety? Therefore Scripture cries out to the ungrateful, the slothful and the craven, telling them that they are “dumb dogs, not able to bark.” To dogs, therefore, is given the ability to bark in defense of their masters and their homes. Thus you should learn to use your voice for the sake of Christ, when ravening wolves attack his sheepfold. Have the word ready on your lips, lest, like a silent watchdog, you may appear because of your unfaithfulness to abandon the post entrusted to you. Such a dog was the friend and companion of an angel. Not without reason did Raphael in the prophetic book3 cause this dog to accompany the son of Tobias when he went on a journey, in order to drive out Asmodeus and thereby confirm the marriage. The demon is driven out as the result of a grateful recognition, and the union is stabilized. And so, under the symbolism of a dumb animal, the angel Raphael, as director of the young man Tobias whom he had agreed to protect, was able to arouse sentiments of gratitude in him.

[AD 735] Bede on Tobit 6:1-18
Therefore, Tobias set out, and the dog followed him. When the Lord came to save the nations, his holy preachers followed in his footsteps, because they fulfilled what he commanded: "Go and teach all nations" (Matt. XXVIII). Indeed, the Lord first filled the house of Cornelius with the Holy Spirit, and then Peter washed it with water (Acts X). The doctors are called dogs because they defend the house, substance, and spiritual sheep of their Creator from thieves and beasts, that is, from unclean spirits and heretical men.

Tobias, guided by the angel, stayed the first night near the river Tigris; and he went out to wash his feet, and behold, a huge fish came out to devour him. Here again, the sacrament of the Lord's passion is more clearly signified. The huge fish, which was killed by Tobias when it attempted to devour him, as taught by the angel, represents the ancient devourer of the human race, that is, the devil, who, while seeking the death of our Redeemer in the flesh, was captured by the power of divinity. The river Tigris, named for its swift current from the swift beast, the tiger, indicates the course of our death and mortality. In which the huge fish was hiding, because the invisible seducer of the human race held dominion over death. Tobias stayed by the streams of the Tigris, because the Lord, appearing in the world, lived among sinners and mortals, but the wave of sin did not touch him, nor did the prince of darkness find anything of his own in him. Tobias went out into the river to wash his feet; and the Lord accepted death, to which he owed nothing, so that he might cleanse all the faithful, namely his members, from the contagion of sin and death. The fish came upon Tobias, desiring to devour him; and the devil came to the Lord suffering on the cross, seeking if he might find any sin in his soul.

Terrified by the fish, Tobias cried out with a loud voice, saying: "Lord, it attacks me." And the Lord, at the imminent moment of death, began to fear and be troubled, not fearing the devil, but the death that entered the world through the envy of the devil (Wis. II), trembling with the natural frailty of the flesh; hence he prayed that if possible, the hour might pass from him (Mark XII). And he said: "Abba, Father, all things are possible for you, remove this cup from me; yet not what I will, but what you will" (Luke XXII).

The angel said to Tobias, "Take hold of the fish's gill, and draw it to you." The Lord took hold of the devil, and he who sought to capture him in death was himself captured and defeated by dying. He took hold of its gill so that with the right hand of his power he might separate the most wicked head from the deceived body, that is, he might remove the wickedness of the ancient enemy from those whom he had wrongly joined to himself and made almost one body with himself, and insert them into the body of his church as a kind Redeemer. For the fish has a gill at the boundary between its head and body. Just as our Lord is the head of his church, and the church is his body; so the devil is the head of all the wicked, and all the wicked are his body and members. Therefore, the Lord took hold of the gill of the huge fish, and drew it to himself, and cast it on dry land, because by breaking the power of the devil, he confidently led them openly and rescued from the power of darkness those whom he foreknew to be children of light.

When he had done this, the fish began to thrash about on dry land before his feet. When the Lord, having overcome the wickedness of the malignant enemy, brought it to light and made it manifest to all, the proud devil still endeavored to stir up persecution against His chosen ones, who are His feet; because through them the Lord walks on earth, who reigns above all in heaven.

Then the angel of the Lord said to him, "Gut this fish, etc." The Lord gutted the fish when He revealed the devil's wickedness more broadly to the saints, and as if eviscerated the hidden schemes of his plots. He kept the heart for Himself, because He wrapped up his cunning in the holy books; about which it is written, "Now the serpent was more cunning than any beast of the field" (Gen. III). And Paul also said of this heart: "For we are not ignorant of his designs" (II Cor. II). He also kept the gall, because He wanted it to be written and preserved how furiously he raged against the human race for the sake of caution. He also kept the liver, because He deigned to make known to us, through the doctors of truth, the devil's malicious plans against us. They say that by the heat and power of the liver, the hidden parts of consumed food are digested and reach the stage of digestion. When we diligently inquire by careful thought how the things we intend to do should be accomplished, it is as if we digest the food taken into the stomach by the heat of the liver.

These things are necessary for useful medicines. The cunning and malice of the devil, the ancient enemy, known to us, help towards healing; for the more we thoroughly investigate them, the more cautiously we avoid them.

Tobias roasted the fish’s flesh, etc. Whatever they took from the fish signifies those who are transferred from being members of the devil to being members of Christ, that is, those converted from unbelief to faith. What they left behind signifies, on the other hand, those who, having heard the word of God, prefer to remain among the dead and decaying members of their deceiver rather than return to the fellowship of the Savior. He roasted its flesh in those whom he found carnal, but by the fire of his love, he made them spiritual and strong. Indeed, the Holy Spirit descended on the apostles in the appearance of fire. "The rest," he says, "they salted," which pertains spiritually to the teachers, to whom it is said: "You are the salt of the earth" (Matt. V). They salted them, that is, Tobias and the angel; for the same Mediator between God and men both taught the apostles humanly by speaking and divinely gave them the salt of wisdom in their hearts.

They took with them what was sufficient for them, etc. Because the Lord gathered enough from Judea into the faith, who were sufficient for the example of living or for the ministry of preaching, until He also laid the foundations of the Church among the Gentiles.

The angel suggests to Tobias that, upon entering the house of Raguel, he should ask for his daughter Sarah as his wife. Raguel signifies the people of the Gentiles, whom the Lord deigned to visit through His preachers, so that He might take a bride for Himself from among them, that is, make the Gentiles His Church. The name Sarah is also fitting for the Church, due to Sarah, the wife of the patriarch Abraham, who gave birth to Isaac, the son of promise, that is, the free people of the Church. The name Raguel, which means "God's pasture" or "God's friend," designates the people who, having overcome the deceit of the devil, bind themselves and theirs to the fellowship of the Lord, who can say: "The Lord is my shepherd, I shall not want" (Psalm 23). And who deserves to hear: "I no longer call you servants, but friends" (John 15).

"To you belongs all his substance." And the Father to the Son: "Ask of me, and I will make the nations your heritage, and the ends of the earth your possession" (Psalm 2).