If I believe that Yeshua (Jesus) is the Messiah, am I still Jewish?
Certainly. If you are born a Jew, you will always be a Jew. For example, former Secretary of State Madeleine Albright discovered late in life that she was born Jewish. Even though she had been raised a practicing Catholic, Jewish halakhah (law) still regards her (and any of her children) as Jewish. Actually, believing in Yeshua is very Jewish, because “Messiah” (Moshiach) is a Jewish idea! Yeshua himself was a Jew (since his mother Miriam was a Jew), he lived as a Jew, and he said that “Salvation is of the Jews” (John 4:22). A Messianic Jew lives out his calling, from birth, to follow Yeshua as a Jew.