Chrysostom: But, my admirable and excellent Sir, this is the very reason why I took the precaution of saying that it was a good thing to employ this kind of deceit, not only in war, and in dealing with enemies, but also in peace, and in dealing with our dearest friends. For as a proof that it is beneficial not only to the deceivers, but also to those who are deceived; if you go to any of the physicians and ask them how they relieve their patients from disease, they will tell you that they do not depend upon their professional skill alone, but sometimes conduct the sick to health by availing themselves of deceit, and blending the assistance which they derive from it with their art.
For when the waywardness of the patient and the obstinacy of the complaint baffle the counsels of the physicians, it is then necessary to put on the mask of deceit in order that, as on the stage, they may be able to hide what really takes place. But, if you please, I will relate to you one instance of stratagem out of many which I have heard of being contrived by the sons of the healing art. A man was once suddenly attacked by a fever of great severity; the burning heat increased, and the patient rejected the remedies which could have reduced it and craved for a draught of pure wine, passionately entreating all who approached to give it him and enable him to satiate this deadly craving— I say deadly, for if any one had gratified this request he would not only have exasperated the fever, but also have driven the unhappy man frantic.
Thereupon, professional skill being baffled, and at the end of its resources and utterly thrown away, stratagem stepped in and displayed its power in the way which I will now relate. For the physician took an earthen cup brought straight out of the furnace, and having steeped it in wine, then drew it out empty, filled it with water, and, having ordered the chamber where the sick man lay to be darkened with curtains that the light might not reveal the trick, he gave it him to drink, pretending that it was filled with undiluted wine.
And the man, before he had taken it in his hands, being deceived by the smell, did not wait to examine what was given him, but convinced by the odor, and deceived by the darkness, eagerly gulped down the draught, and being satiated with it immediately shook off the feeling of suffocation and escaped the imminent peril. Do you see the advantage of deceit? And if any one were to reckon up all the tricks of physicians the list would run on to an indefinite length. And not only those who heal the body but those also who attend to the diseases of the soul may be found continually making use of this remedy.
Thus the blessed Paul attracted those multitudes of Jews: with this purpose he circumcised Timothy, although he warned the Galatians in his letter that Christ would not profit those who were circumcised. For this cause he submitted to the law, although he reckoned the righteousness which came from the law but loss after receiving the faith in Christ. For great is the value of deceit, provided it be not introduced with a mischievous intention. In fact action of this kind ought not to be called deceit, but rather a kind of good management, cleverness and skill, capable of finding out ways where resources fail, and making up for the defects of the mind.
For I would not call Phinees a murderer, although he slew two human beings with one stroke: nor yet Elias after the slaughter of the 100 soldiers, and the captain, and the torrents of blood which he caused to be shed by the destruction of those who sacrificed to devils. For if we were to concede this, and to examine the bare deeds in themselves apart from the intention of the doers, one might if he pleased judge Abraham guilty of child-murder and accuse his grandson and descendant of wickedness and guile.
For the one got possession of the birthright, and the other transferred the wealth of the Egyptians to the host of the Israelites. But this is not the case: away with the audacious thought! For we not only acquit them of blame, but also admire them because of these things, since even God commended them for the same. For that man would fairly deserve to be called a deceiver who made an unrighteous use of the practice, not one who did so with a salutary purpose. And often it is necessary to deceive, and to do the greatest benefits by means of this device, whereas he who has gone by a straight course has done great mischief to the person whom he has not deceived.
Source: On the Priesthood (New Advent)