A lawyer - This does nor mean one that "practiced" law, as among us, but one learned or skilled in the law of Moses.
Mark calls him "one of the scribes." This means the same thing. The scribes were men of learning - particularly men skilled in the law of Moses. This lawyer had heard Jesus reasoning with the Sadducees, and perceived that he had put them to silence. He was evidently supposed by the Pharisees to be better qualified to hold a debate with him than the Sadducees were, and they had therefore put him forward for that purpose. This man was probably of a candid turn of mind; perhaps willing to know the truth, and not entering very fully into their malicious intentions, but acting as their agent, Mark 12:34.
Tempting him - Trying him. Proposing a question to test his knowledge of the law.

A lawyer - Νομικος, a teacher of the law. What is called lawyer, in the common translation, conveys a wrong idea to most readers: my old MS. renders the word in the same way I have done. These teachers of the law were the same as the scribes, or what Dr. Wotton calls letter-men, whom he supposes to be the same as the Karaites, a sect of the Jews who rejected all the traditions of the elders, and admitted nothing but the written word. See Wotton's Mishna, vol. i. p. 78. These are allowed to have kept more closely to the spiritual meaning of the law and prophets than the Pharisees did; and hence the question proposed by the lawyer, (Mark, Mark 12:28, calls him one of the scribes), or Karaite, was of a more spiritual or refined nature than any of the preceding.

Then (o) one of them, [which was] a lawyer, asked [him a question], tempting him, and saying,
(o) A scribe, so it says in (Mark 12:28). To understand what a scribe is, See Matthew 2:4

Then one of them, which was a lawyer,.... Or that was "learned", or "skilful in the law", as the Syriac and Persic versions, and Munster's Hebrew Gospel read. The Ethiopic version calls him, "a Scribe of the city", of the city of Jerusalem; but I do not meet with any such particular officer, or any such office peculiar to a single man any where: mention is made of "the Scribes of the people" in Matthew 2:4 and this man was one of them, one that interpreted the law to the people, either in the schools, or in the synagogues, or both; and Mark expressly calls him a "Scribe": and so the Arabic version renders the word here; and from hence it may be concluded that the lawyers and Scribes were the same sort of persons. This man was by sect a Pharisee, and by his office a Scribe; or interpreter of the law, and suitable to his office and character,
asked him a question, tempting him, and saying: he put a difficult and knotty question to him, and thereby making a trial of his knowledge and understanding of the law; and laying a snare for him, to entrap him if he could, and expose him to the people, as a very ignorant man: and delivered it in the following form.

A lawyer. An expounder of the law of Moses. A scribe (see Mark).
Tempting him. Trying him.

A scribe asking him a question, trying him - Not, as it seems, with any ill design: but barely to make a farther trial of that wisdom, which he had shown in silencing the Sadducees.