Genesis 18:1

Abraham, ready to receive strangers, faithful towards God, devoted in ministering, quick in his service, saw the Trinity in a type; he added religious duty to hospitality, when beholding Three he worshipped One, and preserving the distinction of the Persons, yet addressed one Lord, he offered to Three the honour of his gift, while acknowledging one Power. It was not learning but grace which spoke in him, and he believed better what he had not learnt than we who have learnt. No one had falsified the representation of the truth, and so he sees Three, but worships the Unity. He brings forth three measures of fine meal, and slays one victim, considering that one sacrifice is sufficient, but a triple gift; one victim, an offering of three.

But let us see what this tree represents, under which Abraham stood and provided a meal for the Lord and the angels. "Under the tree of Mamre" the text says. Mamre in our language is translated "vision" or "sharpness of sight." Do you see what kind of place it is where the Lord can have a meal? Abraham's vision and sharpness of sight pleased the Lord. For he was pure in heart so that he could see God. In such a place, therefore, and in such a heart the Lord can have a meal with his angels. In fact, earlier prophets were called seers.

Now where did this happen? "Near the holm-oak of Mamre," which in Latin is interpreted as "vision" or "discernment." Do you see what kind of a place it is in which the Lord can have a feast?

Thus the Lord God is said to have appeared as a common man to Abraham while he was seated by the oak of Mamre. But [Abraham] immediately fell down, although he saw a man with his eyes, and worshiped him as God, besought him as Lord and confessed that he was not ignorant as to who he was, using these very words, "O Lord, judge of all the earth, will you not judge righteously?" For if it should be unreasonable to suppose that the unbegotten and immutable substance of God the Almighty was changed into the form of man and, in turn, that the eyes of the beholders were deceived by the phantasm of something created and that such things were falsely invented by Scripture, who else could be proclaimed God and the Lord who judges all the earth and judges righteously, appearing in the shape of a man—if it be not proper to call him the first cause of all things—than his preexistent Word alone?

God appeared again to Abraham at the oak of Mamre in three men, who it is not to be doubted were angels, although some think that one of them was Christ and assert that he was visible before he put on flesh. Now it belongs to the divine power and invisible, incorporeal and incommunicable nature, without changing itself at all, to appear even to mortals, not by what it is but by what is subject to it. And what is not subject to it? Yet if they try to establish that one of these three was Christ by the fact that although he saw three, he addressed the Lord in the singular, as it is written, "He lifted up his eyes and looked, and behold, three men stood in front of him. When he saw them, he ran from the tent door to meet them, and bowed himself to the earth and said, 'My lord, if I have found favor in your sight.' " Why do they not refer also to this, that when two of them came to destroy the Sodomites, while Abraham still spoke to one, calling him Lord and interceding that he would not destroy the righteous along with the wicked in Sodom, Lot received these two in such a way that he too in his conversation with them addressed the Lord in the singular? For after saying to them in the plural, "My lords, turn aside, I pray you, to your servant's house," yet it is afterward said, "So [the angels] seized him and his hand, [because] the Lord [was] merciful to him, and they brought him forth and set him outside the city.