Pastor R___ said that Abraham was a Gentile and that he became a Jew after he was circumcised.
I assume your question is whether or not the Pastor is correct. Actually, he is partly right and partly wrong.
Yes, Abraham was a Gentile. God called him from among the pagan nations. Joshua referred to this in his farewell to the people of Israel.
But if serving the LORD seems undesirable to you, then choose for yourselves this day whom you will serve, whether the gods your forefathers served beyond the River, or the gods of the Amorites, in whose land you are living. But as for me and my household, we will serve the LORD. – Joshua 24:15
The forefathers here were the family of Abraham, whom God called Abraham to leave. The River was the Euphrates River, which ran through Ur, the city in which Abraham had lived. Joshua is challenging the nation of Israel to live up to the Covenant God made with Abraham when He called him. He said he and his family would serve the LORD – and the people also responded with vigor: “We will serve the LORD!”
Abraham came from a pagan, Gentile family, and was serving God before he was circumcised.
However, Abraham was neither a Jew nor an Israelite. The name “Jew” comes from the name “Judah.” Judah was a great-grandson of Abraham, born after Abraham had died. Judah was the beginning of the tribe of Judah, which was the tribe of King David and later of Jesus. All Israelites came to be known as “Jews” because of the prominence of that tribe. In fact, the country of the Jews was known as Judea in the time of Jesus, a name that also came from Judah.
In the same way, Abraham was not an Israelite. Israel is the name God gave to Abraham’s grandson, Jacob (again, after Abraham was dead). The Israelites were the descendants of Israel. One of the Twelve Tribes of Israel was the tribe of Judah, who eventually gave their name to all of the nation.
So, Abraham was the Father of the nation of Israel and the Father of the Jews – but he himself was neither Israelite not Jew. Those designations came later.
In fact, the Covenant God made with Abraham that called him out of his pagan background came more than a decade before Abraham was circumcised. Circumcision was a sign of that covenant. All of Abraham’s children were circumcised, not just the children of Isaac, the father of Jacob. Both Abraham and Isaac were the fathers of other nations than Israel. Some of those other nations, in particular the Arab nations who descend from Ishmael (a son of Abraham through a concubine, Hagar) still practice circumcision today. They, however, have nothing to do with the Jews. Indeed, they hate them very much.
In the New Testament, Paul makes the point that all who believe (trust) God are the children of Abraham, whether they are circumcised or not.
[Abraham] received the sign of circumcision, a seal of the righteousness that he had by faith while he was still uncircumcised. So then, he is the father of all who believe but have not been circumcised, in order that righteousness might be credited to them. And he is also the father of the circumcised who not only are circumcised but who also walk in the footsteps of the faith that our father Abraham had before he was circumcised. – Romans 4:11-12 (see the surrounding discussion as well)
All who trust God for their salvation today are also children of Abraham because they share his faith in God.