3 These few words, as the shortness of the time allowed me, would I speak concerning the Lord who was pleased to be fed in the flesh, while He feeds in the spirit: let us now come to the subject which I have proposed concerning unity. Martha, who was arranging and preparing to feed the Lord, was occupied about much serving. Mary her sister chose rather to be fed by the Lord. She in a manner deserted her sister who was toiling about much serving, and she sat herself at the Lord's feet, and in stillness heard His word.
Her most faithful ear had heard already; “Be still, and see that I am the Lord.” Martha was troubled, Mary was feasting; the one was arranging many things, the other had her eyes upon the One. Both occupations were good; but yet as to which was the better, what shall we say? We have One whom we may ask, let us give ear together. Which was the better, we heard now when the lesson was read, and let us hear again as I repeat it. Martha appeals to her Guest, lays the request of her pious complaints before the Judge, that her sister had deserted her, and neglected to assist her when she was so busied in her serving.
Without any answer from Mary, yet in her presence, the Lord gives judgment. Mary preferred as in repose to commit her cause to the Judge, and had no mind to busy herself in making answer. For if she were to be getting ready words to answer, she must remit her earnest attention to hear. Therefore the Lord answered, who was in no difficulty for words, in that He was the Word. What then did He say? “Martha, Martha.” The repetition of the name is a token of love, or perhaps of exciting attention; she is named twice, that she might give the more attentive heed.
“Martha, Martha,” hear: “You are occupied about many things: but one thing is needful;” for so means unum opus est, not “one work,” that is, one single work, but one is needful, is expedient, is necessary, which one thing Mary had chosen.
Source: Sermons on Selected Lessons of the New Testament (New Advent)