Jerome

The young man speaks false, for if he had fulfilled that which was afterwards placed among the commandments, Thou shalt love thy neighbour as thyself, how was it that when he heard, Go and sell all that thou hast, and give to the poor, he went away sorrowful?

Catena Aurea by Aquinas ↗

Bede

He said, "All these I have kept from my youth." Jesus, hearing this, said to him, "You still lack one thing." This prince is not to be thought a liar when he claimed he kept the commandments of the law, but he confessed simply as he had lived. Because if he were guilty of the charge of lying, the evangelist Mark would not have added, writing about him: "Jesus looking at him, loved him, and said to him, 'You lack one thing: Go, sell all that you have' (Mark 10), and so on. …

On the Gospel of Luke ↗

Bonaventure

And since these commandments do not justify through simple knowledge, but through experiential observance, therefore the Lord was not asking about simple knowledge when He said: You know the commandments, but about experiential knowledge, because, as is said in Romans 2, "not the hearers of the law, but the doers of the law are just before God." Therefore this lawyer responds as one who understands, when it follows: Who said: All these things I have kept from my youth: and this indeed according to the counsel of the Wise Man in …

Commentary on Luke, Chapter 18 ↗