He was a Jewish Catholic.
The very Catholic Cardinal Aron Jean-Marie Lustiger, former Archbishop of Paris, was an Ashkenazi Jew. He was clearly Catholic, but if you asked him his ethnicity, he would say straight up that he was Jewish.
Why could one not be both?
I'm sure there's a number of reasons.
Romans 3 Paul explains about being Jewish and how faith and works go together. Faith saves but works is a demonstration of that faith. In Romans 3:31 he shows that the Law is still integral in the Christian walk. All of the Apostles were Sabbatarians. Did they preach on the Sabbath only? No, but they kept the Sabbath day holy because that is Gods Law. Gods Law does not save but only points out sin. It is a measurement to judge how far from perfection we are and how far we have to go.
It is impossible to follow the Law without a new heart. You must want to please God and not yourself or your Church friends. That is where the Holy Spirit is to come into your life to help fulfill Gods Law as He wishes. That's what the New Covenant is all about.
“He was a Jewish Catholic.”
LOL! Maybe Jewish catholic, but not Jewish Catholic.