The GT was written by anonymous authors and modified heavily over the years by various "scribes." Ergo no one knows IF there was a Matthew who was a Jew or not. Based on the anti-Jewish fervor it seems unlikely.

A person who is born Jewish or who converts halachally (under Jewish law) is forever a Jew. We made a contract with G-d at Sinai, and that can't be undone. Therefore if a Jew (by birth or conversion) converts to Xianity they are most definitely an apostate and endangering their immortal soul.

The good news is that at any time one may repent and return to G-d.

סופי

And everything that Sarah tells you, listen to her voice. Bereshit (Genesis) 21:12