But one thing is needful: and Mary hath chosen that good part, which shall not be taken away from her. – Luke 10:42

We have no difficulty whatever in deciding what the one thing is. We are not allowed to say that it is the Saviour, for He is not a thing; and we are not permitted to say that it is attention to our own salvation, for although that would be true, it is not mentioned in the context. The one thing needful evidently is that which Mary chose-that good part which should not be taken away from her. Very clearly this was to sit at Jesus’ feet and hear His word. This and nothing less, this and nothing more.

The mere posture of sitting down and listening to the Saviour’s word was nothing in itself: it was that which it indicated. It indicated, in Mary’s case, a readiness to believe what the Saviour taught, to accept and to obey-nay to delight in, the precepts which fell from His lips. And this is the one thing needful-absolutely needful; for no rebel can enter the kingdom of heaven with the weapons of rebellion in his hands. We cannot know Christ while we resist Christ: we must be reconciled to His gentle sway, and confess that He is Lord, to the glory of God the Father.

To sit at Jesus’ feet implies faith as well as submission. Mary believed in what Jesus said, and, therefore, sat there to be taught by Him. It is absolutely necessary that we have faith in the Lord Jesus Christ, in His power as God and man, in His death as being expiatory, in His crucifixion as being a sacrifice for our sins. We must trust Him for time and eternity, in all His relationships as Prophet, Priest, and King. We must rely on Him; He must be our hope, our salvation, our all in all. This one thing is absolutely necessary: without it we are undone. A believing submission, and a submissive faith in Jesus we must have, or perish. ~ C.H. Spurgeon

https://www.blueletterbible.org/comm/spurgeon_charles/sermons/1015.cfm