I’m doing some research on Judaism. Could a Jewish man marry outside the religion, like marry a Baptist, and still be considered a practicing Jew?
No, for an educated Jew to do so would be akin to throwing a Torah scroll on the ground and trampling upon it. It would cut him off from his ancestors, his children would not be Jewish, among other consequences.
For an uneducated Jew to do so would have the same consequences, but he would be regarded by the community with pity, because he didn’t know better.
Bear in mind that the above statements have no bearing on the Jewish view of the righteousness of the Baptist. She may be a fine person, even a tremendously spiritual person; but Jewish civilization is predicated on the belief that we have a Divine mission to perpetuate the Torah as a People.