Peter is the apostle to the circumsision. It seems that Peter is refering to a letter that Paul wrote to them. and links it to other scripture.
Peter’s epistles actually were not written to the circumcision.
I know, most Thomas Nelson Bibles have a preamble that declares that they were written to “Jewish believers,” but a careful read of the beginning of the first epistle shows that they were actually written to the “strangers” of the dispersion of northern Israel, and the author of the preamble was simply ignorant of the fact that Jews (House of Judah) are not a part of the House of Israel.