To be expert - To be skilled or well acquainted.
In all customs - Rites, institutions, laws, etc. Everything pertaining to the Mosaic ritual, etc.
And questions - Subjects of debate, and of various opinions. The inquiries which had existed between the Pharisees, Sadducees, scribes, etc. Paul could say this of Agrippa without falsehood or flattery. Agrippa was a Jew; he had passed much of his time in the kingdom over which he presided; and though he had spent the early part of his life chiefly at Rome, yet it was natural that he should make himself acquainted with the religion of his fathers. Paul did not know how to flatter people, but he was not unwilling to state the truth, and to commend people as far as truth would permit.
Wherefore - On this account; because you are acquainted with those customs. The Romans, who regarded those customs as superstitious, and those questions as matters to be treated with contempt, could not listen to their discussion with patience. Agrippa, who knew their real importance, would be disposed to lend to all inquiries respecting them a patient attention.

Especially, because I know thee to be expert in all customs,.... Rites and ceremonies of the Jews, whether enjoined by the law of Moses, or by the elders, fathers, and wise men:
and questions which are among the Jews; concerning angels, spirits, and the resurrection of the dead; which were moved and agitated between the Sadducees and Pharisees; and a multitude of others, which were disputed between the schools of Hillell and Shammai, of which their Misna and Talmud are full, and with these Agrippa was well acquainted; and to their rites and customs he conformed, of which we have some instances recorded in their writings: when they went with their firstfruits to Jerusalem (w),
"a pipe sounded before them till they came to the mountain of the house, and when they came to the mountain of the house (the temple), even King Agrippa carried the basket upon his shoulder, and went in till he came to the court.''
So concerning the reading of the law by a king, they give this following account (x):
"a king stands and takes (the book of the law), and reads sitting; King Agrippa stood and took it, and read standing, and the wise men praised him; and when he came to that passage, Deuteronomy 17:15 "Thou mayest not set a stranger over thee", his eyes flowed with tears; they said unto him, fear not, Agrippa, thou art our brother.''
Some of their writers say (y), this was a piece of flattery in them: they also elsewhere commend him for his modesty and humility (z);
"according to the tradition of the doctors, when persons attending a funeral met a bride (with her retinue), the former gave way, and both to a king of Israel, when they met him; but they say concerning King Agrippa, that he met a bride, and gave way, and they praised him.''
And whereas it was forbidden to eat on the eve of the passover, before the Minchah, though ever so little, that they might eat the unleavened bread with appetite (a); it is observed, that even King Agrippa, who was used to eat at the ninth hour, that day did not eat till it was dark (b): so that from hence it appears, that King Agrippa was famous for his exact knowledge and observance of the customs and manners of the Jews, and which was well known, and was by the apostle:
wherefore I beseech thee to hear me patiently; since he was charged with a breach of the laws and customs of the Jews; and his defence would proceed upon things which Agrippa was not altogether ignorant of.
(w) Misn. Biccurim, c. 3. sect. 4. (x) Misn. Sota, c. 7. sect. 8. (y) Maimon. in ib. & Moses Kotsensis Mitzvot Tora, pr. neg. 221. (z) T. Bab. Cetubot, fol. 17. 1. (a) Maimon. Chametz Umetza, c. 6. sect. 12. (b) T. Bab. Pesachim, fol. 107. 2.

I know thee to be expert, &c.--His father was zealous for the law, and he himself had the office of president of the temple and its treasures, and the appointment of the high priest [JOSEPHUS, Antiquities, 20.1.3].
hear me patiently--The idea of "indulgently" is also conveyed.

Who art accurately acquainted - Which Festus was not; with the customs - In practical matters; and questions - In speculative. This word Festus had used in the absence of Paul, Acts 25:19, who, by the Divine leading, repeats and explains it. Agrippa had had peculiar advantages for an accurate knowledge of the Jewish customs and questions, from his education under his father Herod, and his long abode at Jerusalem. Nothing can be imagined more suitable or more graceful, than this whole discourse of Paul before Agrippa; in which the seriousness of the Christian, the boldness of the apostle, and the politeness of the gentleman and the scholar, appear in a most beautiful contrast, or rather a most happy union.