And I will fetch a morsel of bread,—a modest description of what proved a sumptuous repast (vide Genesis 18:6, Genesis 18:8)—and comfort ye your hearts;—literally, strengthen or support them, i.e. by eating and drinking ( 19:5; 1 Kings 21:7)—after that ye shall pass on: for therefore— כִּי־עַל־כֵּן introduces the ground of what has already been stated, something like quando quidem, forasmuch as, since, or because (Kalisch), and not = עַל־כֵּש־כִּי, for this cause that, or "because for this purpose" (Keil)—are ye come to (literally, have ye passed before) thy servant.
The patriarch's meaning is not that they had come with the design of receiving his gifts (LXX; A.V.), but either that, unconsciously to them, God had ordered their journey so as to give him this opportunity (Calvin, Bush, Wordsworth, 'Speaker's Commentary,' Keil), or perhaps simply that since they had passed by his tent they should suffer him to accord them entertainment (Kalisch, Rosenmüller).
And they said, So do, as thou but said. Therefore we must believe that Abraham washed the men's feet, and they did eat (Genesis 18:8). Here is a mystery (Wordsworth).