What is the evidence for Paul's persecution? Firstly, there are the three "confessions" of persecution in the Pauline letters:
3 For I handed down to you as of first importance what I also received, that Christ died for our sins according to the Scriptures, 4 and that He was buried, and that He was raised on the third day according to the Scriptures, 5 and that He appeared to Cephas, then to the twelve. 6 After that He appeared to more than five hundred brothers and sisters at one time, most of whom remain until now, but some have fallen asleep; 7 then He appeared to James, then to all the apostles; 8 and last of all, as to one untimely born, He appeared to me also. 9 For I am the least of the apostles, and not fit to be called an apostle, because I persecuted the church of God. 10 But by the grace of God I am what I am, and His grace toward me did not prove vain; but I labored even more than all of them, yet not I, but the grace of God with me. 11 Whether then it was I or they, so we preach and so you believed.
13 For you have heard of my former way of life in Judaism, how I used to persecute the church of God beyond measure and tried to destroy it; 14 and I was advancing in Judaism beyond many of my contemporaries among my countrymen, being more extremely zealous for my ancestral traditions. 15 But when He who had set me apart even from my mother’s womb and called me through His grace was pleased 16 to reveal His Son in me so that I might preach Him among the Gentiles, I did not immediately consult with flesh and blood, 17 nor did I go up to Jerusalem to those who were apostles before me; but I went away to Arabia, and returned once more to Damascus.
18 Then three years later I went up to Jerusalem to become acquainted with Cephas, and stayed with him for fifteen days. 19 But I did not see another one of the apostles except James, the Lord’s brother. 20 (Now in what I am writing to you, I assure you before God that I am not lying.) 21 Then I went into the regions of Syria and Cilicia. 22 I was still unknown by sight to the churches of Judea which are in Christ; 23 but they only kept hearing, “The man who once persecuted us is now preaching the faith which he once tried to destroy.” 24 And they were glorifying God because of me.
2 Beware of the dogs, beware of the evil workers, beware of the false circumcision; 3 for we are the true circumcision, who worship in the Spirit of God and take pride in Christ Jesus, and put no confidence in the flesh, 4 although I myself could boast as having confidence even in the flesh. If anyone else thinks he is confident in the flesh, I have more reason: 5 circumcised the eighth day, of the nation of Israel, of the tribe of Benjamin, a Hebrew of Hebrews; as to the Law, a Pharisee; 6 as to zeal, a persecutor of the church; as to the righteousness which is in the Law, found blameless.
Of these, 1 Cor 15:5-10 is clearly an interpolation. This is an obvious anti-Marcionite statement that was later added. I'm not going to go into the case for it here, as it has been discussed many times.
Philippians 3:6 I find very suspect, as I've discussed in this recent thread: viewtopic.php?f=3&t=8140
But this still leaves us with the statements in Galatians. Before tackling Galatians, let's look at what Acts of the Apostles has to say.
1 Now Saul approved of putting Stephen to death.
And on that day a great persecution began against the church in Jerusalem, and they were all scattered throughout the regions of Judea and Samaria, except for the apostles. 2 Some devout men buried Stephen, and mourned loudly for him. 3 But Saul began ravaging the church, entering house after house; and he would drag away men and women and put them in prison.
1 Now Saul, still breathing threats and murder against the disciples of the Lord, went to the high priest, 2 and asked for letters from him to the synagogues in Damascus, so that if he found any belonging to the Way, whether men or women, he might bring them in shackles to Jerusalem. 3 Now as he was traveling, it happened that he was approaching Damascus, and suddenly a light from heaven flashed around him; 4 and he fell to the ground and heard a voice saying to him, “Saul, Saul, why are you persecuting Me?” 5 And he said, “Who are You, Lord?” And He said, “I am Jesus whom you are persecuting, 6 but get up and enter the city, and it will be told to you what you must do.” 7 The men who traveled with him stood speechless, hearing the voice but seeing no one. 8 Saul got up from the ground, and though his eyes were open, he could see nothing; and leading him by the hand, they brought him into Damascus. 9 And for three days he was without sight, and neither ate nor drank.
There is more, but that's the crux of it.
Saul's persecution of the Lord, or of the disciples of the Lord, is very much patterned on the story of David and Saul from 1 Samuel. From 1 Samuel we read:
10 Now it came about on the next day that an evil spirit from God rushed upon Saul, and he raved in the midst of the house while David was playing the harp with his hand, as usual; and a spear was in Saul’s hand. 11 Then Saul hurled the spear, for he thought, “I will pin David to the wall.” But David escaped from his presence, twice.
12 Now Saul was afraid of David, because the Lord was with him but had left Saul. 13 So Saul removed him from his presence and appointed him as his commander of a thousand; and he went out and came in before the people. 14 David was successful in all his ways, for the Lord was with him. 15 When Saul saw that he was very successful, he was afraid of him. 16 But all Israel and Judah loved David, for he would go out to battle and return before them.
24 Now when Saul returned from pursuing the Philistines, it was reported to him, saying, “Behold, David is in the wilderness of Engedi.” 2 Then Saul took three thousand chosen men from all Israel and went to search for David and his men in front of the Rocks of the Mountain Goats. 3 And he came to the sheepfolds on the way, where there was a cave; and Saul went in to relieve himself. Now David and his men were sitting in the inner recesses of the cave. 4 Then David’s men said to him, “Behold, this is the day of which the Lord said to you, ‘Behold; I am about to hand your enemy over to you, and you shall do to him as it seems good to you.’” Then David got up and cut off the edge of Saul’s robe secretly. 5 But it came about afterward that David’s conscience bothered him because he had cut off the edge of Saul’s robe. 6 So he said to his men, “Far be it from me because of the Lord that I would do this thing to my lord, the Lord’s anointed, to reach out with my hand against him, since he is the Lord’s anointed.” 7 And David rebuked his men with these words and did not allow them to rise up against Saul. And Saul got up, left the cave, and went on his way.
8 Afterward, however, David got up and went out of the cave, and called after Saul, saying, “My lord the king!” And when Saul looked behind him, David bowed with his face to the ground and prostrated himself. 9 And David said to Saul, “Why do you listen to the words of men who say, ‘Behold, David is seeking to harm you’? 10 Behold, this day your eyes have seen that the Lord had handed you over to me today in the cave, and someone said to kill you, but I spared you; and I said, ‘I will not reach out with my hand against my lord, because he is the Lord’s anointed.’ 11 So, my father, look! Indeed, look at the edge of your robe in my hand! For by the fact that I cut off the edge of your robe but did not kill you, know and understand that there is no evil or rebellion in my hands, and I have not sinned against you, though you are lying in wait for my life, to take it. 12 May the Lord judge between you and me, and may the Lord take vengeance on you for me; but my hand shall not be against you. 13 As the proverb of the ancients says, ‘Out of the wicked comes wickedness’; but my hand shall not be against you. 14 After whom has the king of Israel gone out? Whom are you pursuing? A dead dog, a single flea? 15 May the Lord therefore be judge and decide between you and me; and may He see and plead my cause and save me from your hand.”
16 When David had finished speaking these words to Saul, Saul said, “Is this your voice, my son David?” Then Saul raised his voice and wept. 17 And he said to David, “You are more righteous than I; for you have dealt well with me, while I have dealt maliciously with you. 18 You have declared today that you have done good to me, that the Lord handed me over to you and yet you did not kill me. 19 Though if a man finds his enemy, will he let him go away unharmed? May the Lord therefore reward you with good in return for what you have done to me this day. 20 Now, behold, I know that you will certainly be king, and that the kingdom of Israel will be established in your hand.
This symbolism was not lost on early apologists. In fact, the relationship between Saul and Paul was central to Tertullian's defense of the correctness of the orthodox Pauline letters vs Marcion's letters, due to the alignment of the orthodox Pauline letters with the account in Acts of the Apostles. Tertullian states: https://www.newadvent.org/fathers/03125.htm
Tertullian essentially states that Paul, as described in Acts of the Apostles and the orthodox version of the Pauline letters, was pre-figured in the Jewish scriptures. However, Tertullian knows that Marcionites don't place any value in the Jewish scriptures, so he also appeals to the agreement between the orthodox Pauline letters and Acts of the Apostles. But, this actually lays out the logic of the invention of Paul's persecution.
Marcion did not believe that the Jewish scriptures were valid, or that they were able to foretell the coming of Jesus. And Marcion claimed that Paul was "his" prophet, the prophet of Jesus, who was the son of the Unknown God. The casting of Paul as a persecutor of Jesus' followers, then served to re-identify Paul as an agent of the Jewish Creator God, as opposed to Marcion's supposed Unknown God.
By casting Paul as a persecutor, now Paul could be identified with Jewish prophecy and it could be claimed that his actions were foretold by the Jewish scriptures. Tertullian, lays out as proof that all of this is valid, the fact that the claims of Paul's persecution are attested to in both the orthodox Pauline letters and in Acts of the Apostles. The agreement between those two sources is used to corroborate one another.
This, then, drives back to my proposition, that this agreement was actually the product of the editing of the Pauline letters by the writer of Acts of the Apostles.
We know that the casting of Paul as a persecutor served an apologetic goal. Thus, it is my strong suspicion that all accounts of Paul as persecutor are post-Marcionite inventions, created in reaction to Marcionism. This would imply that Marcion's version of the Pauline letters contained no claims about Paul having engaged in persecution.
Such a position is not clearly supported by Tertullian's commentary, because Tertullian is at times unclear about what he finds in his version of the letters vs Marcion's. Tertullian does not specifically call out Marcion for having removed all claims as to Paul's persecution, but he does make the case that Marcion's version of the letters are in error and that the orthodox version is correct, in part because of agreements between Acts and the orthodox version of the letters.
The importance of Paul's role as a persecutor was certainly amplified by anti-Marcionite Christians because it was seen as evidence that Paul was an agent of the Jewish Creator God as opposed to Marcion's supposed Unknown God. Thus, we have to ask if any claims of persecution in the Pauline letters is authentic, including that found in Galatians.
Is there substantial evidence that Paul was known as a persecutor prior to the mid second century?