While it is true that Paul became known as a servant of God who was a Christian he maintained his ethnicity as a Jew although the religion itself was then seen as being the same with Christianity and not like it is today where the Jews deny that Jesus was their Christ.
ETHNICALLY, Paul was a Hebrew (and no one can change their DNA). However, Paul stressed that IN CHRIST there is no Jew or Gentile... race/ethnicity is entirely irrelevant and in relation to our faith, nonexistent.
Christians did continue to also go to Temple while becoming Christian, but I would think that Paul in his new self of proclaiming the Gospel focused on Christ and would have thus left the religion of Judaism.
Paul was Saul remember that? As when he became the beloved Paul, the Christians he was sent out to persecute. Now he was wanting to save and that proclaiming Christianity in all shapes, sizes, and forms.