And he said, Who art thou, Lord? And the Lord said, I am Jesus whom thou persecutest: [it is] hard for thee to kick against the pricks.
Note 5 at Ac 9:5: Saul knew whom he had been persecuting, and he no doubt had a very good idea who this was talking to him. But as Ac 9:6 says, he was astonished and apparently found it hard to believe that he had been so wrong. However, in the awesome presence of God, he had the good sense to recognize that whoever this was, was Lord.
Note 6 at Ac 9:5: The word used for "pricks" here is the Greek word "KENTRON," and it means "a point ("centre"), i.e. a sting (figuratively, poison) or goad (figuratively, divine impulse)" (Strong's Concordance).
A goad was a pointed stick used for urging on oxen, horses, and other beasts of burden. For Saul to "kick against the pricks," that means that Saul was fighting against the reproof and conviction that the Lord had been giving him. This leaves no doubt that Saul had been under conviction for some time, possibly since the stoning of Stephen.
As mentioned in note 6 at Ac 5:28, sometimes those who protest the loudest are the ones who are the most convicted. Saul's severe persecution against the Christians was just a manifestation of the conflict that was going on inside him. He had been trained to adhere to the Law in the strictest sense, but there was a growing revelation inside him that Jesus had fulfilled the Law.