Complex sentence: Some of the Jewish followers of Sabbatai Zevi (Sabbateans) formally converted to Islam, and later followers of Jacob Frank ("Frankists") formally converted to Christianity, but maintained aspects of their versions of Messianic Judaism.
Split into simpler sentences: Some of the Jewish followers of Sabbatai Zevi (Sabbateans) formally converted to Islam. Later followers of Jacob Frank ("Frankists") formally converted to Christianity, but maintained aspects of practice of their versions of Messianic Judaism.