Kabbalists believed that there is a gap between how mankind can understand God and what God actually is. This unknowable aspect of God is called Ein-Sof.
Kabbalah, meaning ‘tradition’ is a school of Jewish mysticism based on the text, the Zohar, which interpreted the Torah as a way to return to God. Later, Christianity and more modern esoteric Occultists adopted its practices. Read articles about Kabbalah below.