Is the God of the Apostle Paul (who was a Hebrew Christian) a different God from the God of Abraham (who was the first Hebrew)?
There is only one God that is God. Any other that claims to be God is false. (See Isaiah 43:10) The God of the apostle Paul is the God of Abraham. The God of the Old Testament is the God of the New Testament

Isaiah 43:10 Ye are my witnesses, saith the Lord, and my servant whom I have chosen: that ye may know and believe me, and understand that I am he: before me there was no God formed, neither shall there be after me.