Acts 13:9 appears to record the moment Saul's name changes to Paul:
Then Saul, who was also called Paul, filled with the Holy Spirit, looked straight at Elymas and said...
(Prior to this time, he was always called Saul; after this, he is always called Paul.)
Why did he change his name? I guess it was somehow to break with his past and may have been related to Simon being renamed Peter but I don't know more than that.
Is there something significant in the two names?
Interestingly, unlike other biblical characters, we are never told of a "name change" with reference to Paul. Rather, Acts 13:9 tells us that Saul "also is called Paul."
Given that Paul was, according to Acts, born a Roman citizen, it is highly likely that he had a Roman name (Paulus) from birth. At the same time, his parents were devout Jews, and therefore gave him a traditional Hebrew name (Saul).
Why then the switch in usage in Acts? Recall that at his conversion, Paul saw of vision of Jesus and was informed that he would be sent to the Gentiles. The Antioch church was a mixture of Jews and Gentiles, and Paul was probably known predominantly as Saul there, at least at first. The switch in usage comes early on his first missionary journey, in connection with Paul's missionary efforts to convert a Gentile proconsul.
In view of all this, therefore, the probability is that Saul, as "the apostle to the Gentiles," chose to use his Roman name as a matter of connection to his Gentile audience.
1Sa 9:2 And he had a son, whose name [was] Saul, a choice young man, and a goodly: and [there was] not among the children of Israel a goodlier person than he: from his shoulders and upward [he was] higher than any of the people.
Php 3:5 Circumcised the eighth day, of the stock of Israel, [of] the tribe of Benjamin, an Hebrew of the Hebrews; as touching the law, a Pharisee; Php 3:6 Concerning zeal, persecuting the church; touching the righteousness which is in the law, blameless.
Benjaminite
Both Sauls were of the tribe of Benjamin.
Goodly person - Hebrew of Hebrews
Both Sauls were from respectable families.
higher than the people - a Pharisee
The Pharisees sought praise for their outward obedience to the law.
At war
Saul the king offered his own sacrifice not waiting for the prophet in order to zealously go to war. Saul the Pharisee in his own religious zeal did not wait for a word from God to wage war on the church.
Power removed
King Saul lost his kingdom because of his rouge sacrifice. Pharisee Saul lost his power in Israel because of his murderous rampage in the name of God.
Blind
King Saul could no longer discern God, even attempting to gain an audience through a medium. Pharisee Saul was struck blind.
Are these parallels accidental? When Pharisee Saul is renamed to Paul, the parallel with King Saul is broken.
2Co 5:17 Therefore if any man [be] in Christ, [he is] a new creature: old things are passed away; behold, all things are become new.
Saul means 'desired'. In keeping with his new nature Paul means 'small or little'.
Eph 3:8 Unto me, who am less than the least of all saints, is this grace given, that I should preach among the Gentiles the unsearchable riches of Christ;
The significance of these two names must very much depends on whether there really was a name change from Saul to Paul, or whether Paul was given a Jewish name and a Greek or gentile name as a child - not an uncommon practice among the Jews. On the other hand, it was also quite common, even in Palestine, for Jews to be known only by a Greek name. Examples include Aristobulus, Antigonus, Philo of Alexandria, who certainly appear to be from devout Jewish families.
In Acts of the Apostles the change of usage from Saul to Paul is so immediate and dramatic that one could almost expect the author to allude to Saul's decision to change his name if indeed this is what happened. However, I question why Saul would have wanted to abandon his birth name, given that in his epistles Paul consistently describes himself as a proud Jew, going so far as to say that his greatest wish was to convert the Jewish nation (see Romans 11; 1 Corinthians 9:20).
Instead of suggesting that Saul actually changed his name to Paul, Acts 13:9 merely tells us that Saul "also is called Paul." The change of name reference does not begin with Saul's conversion to Christianity on the road to Damascus but much later in Cyprus (Acts 13:9). If Saul had always had both names, there may have been theological reasons for the author of Acts to refer to him as Saul before the miracle that occurred in Cyprus and to call him Paul from that point forward.
Interestingly, Paul never mentions the name Saul in his own epistles, consistently maintaining the name Paul, even when he writes of persecuting the Christians, of his conversion and of his early mission. It is as if he had never used that name or that he was ashamed of it, yet we know he was a proud Jew.
Josephus (Antiquities XX.9.4) talks of a Saulus, a disreputable member of the Herodian family, who used violence with the people and was very ready to plunder those that were weaker than himself. Robert Eisenman, in The Dead Sea Scrolls and the First Christians, sees significance in this name, for the account in Acts of the Apostles. He says (page 242) that directly following the stoning of James, Josephus described Saulus and Costobarus leading a band of thugs in rioting in Jerusalem, which seems to have involved stealing the tithes of the Poorer Priests on behalf of the Rich High Priests. Josephus has Saulus go to see Nero in 66 CE, to inform him of the situation in Palestine, and Eisenman sees a parallel in Acts, where Paul was sent to Nero in 60 CE, when he appealed against trial in Judea. Eisenman suggests the parallels a so strong that Paul and Josephus' Saulus may have been the same man, but this is unlikely because Paul probably went to Rome before 66 CE and there is nothing we know about Paul to suggest that he was in any way a disreputable person.
If the author of Acts intentionally associated the apostle Paul with Saulus, what would the reason have been? Scholars have long noticed irreconcilable differences between Paul’s epistles and corresponding accounts in Acts of the Apostles, and take the position that Acts is not always reliable. I. Howard Marshall (Witness to the Gospel, page 6) says it could be argued that Luke's account of the early church does not accurately mirror reality, even that Luke (or his sources) was presenting a picture of the early church that validated his own theology, going on to say that (if so) this would have been a deliberate procedure. Regardless of the conclusions Marshall reaches, Acts contains subtle comparisons between Paul and the apostle Peter and, for some reason, the author may have wished to make Paul a lesser character than Peter in the early church. A parallel that his early Christian readers might have noticed, between Paul and Saulus, would serve that purpose.
In conclusion, the one reference in Acts does not lead to a clear conclusion that Paul changed his name in Cyprus or at any time as a Christian. Paul never mentions the name Saul in his own epistles, even when it would have been reasonable for him to have done so, and we can find no reason for him to abandon a Jewish birth name, in fact to have done so would have offended the Jews he may have wished to convert. It is therefore possible, but not provable, that Paul was not at any time known as Saul, except in Acts of the Apostles.
Paul's Greek name - Παῦλος - is a transliteration from Latin to Greek of the Latin word for "small" - paulus. The Church Fathers emphasize the aspect of humility in Paul's choice of his new name.
Augustine describes why Saul came to be called Paul in one of his Anti-Pelagian Writings:
Accordingly Paul, who, although he was formerly called Saul, chose this new designation, for no other reason, as it seems to me, than because he would show himself little — the “least of the apostles” — contends with much courage and earnestness against the proud and arrogant, and such as plume themselves on their own works, in order that he may commend the grace of God.
Treatise on the Spirit and the Letter, Chapter 12
In Sermon LI of his Sermons on Selected Lessons of the New Testament, Augustine also writes:
But this Apostle, from Saul, becoming Paul, that is, from being proud, the least of all (for the name of Saul is derived from Saul; but Paul is little; whence in a way interpreting his own name, he says, “I am the least of the Apostles” [1 Corinthians 15:9].
Sermons on Selected Lessons of the New Testament, Sermon XXVII
He goes into much more detail on this subject in one of his Sermons on the New Testament:
Whence was Paul himself, who was first called Saul? That is, first proud, afterwards humble? For when he was Saul, his name was derived from Saul: now Saul was a proud king; and in his reign he persecuted the humble David. So when he who was afterwards Paul, was Saul, he was proud, at that time a persecutor of the innocent, at that time a waster of the Church. For he had received letters from the chief priests (burning as he was with zeal for the synagogue, and persecuting the Christian name), that he might show up whatever Christians he should find, to be punished. While he is on his way, while he is breathing out slaughter, while he is thirsting for blood, he is thrown to the ground by the voice of Christ from heaven the persecutor, he is raised up the preacher. In him was fulfilled that which is written in the Prophet, “I will wound and I will heal.” For that only in man doth God wound, which lifteth itself up against God. He is no unkind physician who opens the swelling, who cuts, or cauterizes the corrupted part. He gives pain, it is true; but he only gives pain, that he may bring the patient on to health. He gives pain; but if he did not, he would do no good. Christ then by one word laid Saul low, and raised up Paul; that is, He laid low the proud, and raised up the humble. For what was the reason of his change of name, that whereas he was afore called Saul, he chose afterwards to be called Paul; but that he acknowledged in himself that the name of Saul when he was a persecutor, had been a name of pride? He chose therefore a humble name; to be called Paul, that is, the least. For Paul is, “the least.” Paul is nothing else but little. And now glorying in this name, and giving us a lesson of humility, he says, “I am the least of the Apostles”.
Sermons on Selected Lessons of the New Testament, Sermon XXVII
Both Augustine and Chrysostom suggest that he was given his new name when his ministry was appointed, much in the same way that the Lord gave Simon bar-Jonah his new name of Peter after his confession in faith that Jesus was the Christ and Son of the Living God [Matthew 16:17-18].
Even that least of Thy apostles, by whose tongue Thou soundedst forth these words, when through his warfare, Paulus the Proconsul, his pride conquered, was made to pass under the easy yoke of Thy Christ, and became a provincial of the great King; he also for his former name Saul, was pleased to be called Paul, in testimony of so great a victory
Augustine, Confessions, VIII.IV.4
“But Saul, who is also Paul” — here his name is changed at the same time that he is ordained, as it was in Peter’s case — “filled with the Holy Ghost, looked upon him, and said, O full of all guile and all villany, thou child of the devil”
John Chrysostom, Homily XV on the Acts of the Apostles
On the other hand, the fact that Luke is talking about Sergius Paulus may have prompted him to mention Paul's other name specifically at this point.
– Tim Gallant May 30 '13 at 13:05