The New Testament records in Acts 13 that Saul changed his name to Paul. Why did he do this? Many commentators have sought to find the answer in the help that Sergius Paulus, governor of Cyprus, provided to Saul and Barnabas on their first journey there. While it is possible that respect and gratitude inspired Saul to take the governor’s name, it seems unlikely based on a number of other intersecting facts.
Scott M. McDonough proposed recently in the Journal of Biblical Literature (Vol. 125, No. 2, pp. 390–391) that Acts 13 holds the key. This chapter contains the only reference in the New Testament to Israel’s first king, Saul, the son of Kish. There is more than one commonality between Paul/Saul and the ancient king. King Saul persecuted David, whom God had anointed to replace him. In a similar way, Saul persecuted the one he later understood to be the true Son of David. By changing his name from Saul to Paul, he distanced himself from the actions and mindset of his namesake.
The choice of the Latin name “Paulus” is instructive as well. The word means “little” or, when referring to a person, “short.” Whether this is a description of Paul’s physical characteristics is not stated, but it has an application to both King Saul and King David. When chosen to be king, Saul was known to stand head and shoulders above his compatriots. But his physical stature was of no consequence to his ability as king. He was only effective in that role when he was “little in [his] own eyes,” or opinion (1 Samuel 15:17). When David was anointed king, his father, Jesse, referred to him as “the youngest” (1 Samuel 16:11). The Hebrew word used here is the same one used earlier to describe Saul’s initial view of himself as “little.” McDonough suggests that the choice of the name “Paulus” is a play on this description of King Saul and his successor, King David. Paul wanted to be known by his namesake’s good quality rather than by his name.
The Bible doesn't say he changed it. Saul was his Hebrew name, but because he was a Roman citizen, he also had a Latin name, which was Paul. The Lord DID change Peter's name, and many people assume that Paul's name was also changed, but it wasn't. Paul's ministry was to the Gentiles, and he may have begun using Paul more because of that, but he already had that name as a citizen of Rome.
We can speculate that this Romanization of his name is an embracing of his mission as an apostle to the Gentiles (see Acts 15). We can also speculate that this new name is a break with the legacy of his covenant-breaking namesake.