Baḥya ibn Paqūda

A rabbinic judge in Muslim Spain who observed that Jewish law had been elaborated in enormous detail on the duties of the limbs — what to do — and hardly at all on the duties of the heart: intention, gratitude, trust, humility, the love of God. So he wrote the missing book, in Arabic, in a vocabulary and a structure drawn openly from the Muslim pietists around him.

The first great crossing, and the one that stuck. Ḥovot ha-Levavot became — and remains — one of the most widely read devotional books in Judaism, studied in yeshivot to this day. Sufi ethics, absorbed so thoroughly that most of its readers have no idea there is a fence anywhere near it.