Kashap means to practice sorcery or witchcraft — possibly derived from a root meaning "to cut" (herbs for potions) or "to whisper" (spells). The noun mekassheph (sorcerer) appears in Exodus 22:18. Sorcery was strictly forbidden in Israel as a direct assault on the unique authority of YHWH over fate, death, and the future.
The prohibition of sorcery is rooted in theology: YHWH alone controls the future, death, fertility, and destiny. To consult sorcerers is to deny His sovereignty and seek unauthorized access to divine knowledge and power. The OT death penalty for mekassheph (Exodus 22:18) reflects how seriously Israel was to take this. In the NT, Simon Magus (Acts 8) and Elymas (Acts 13) represent the ongoing conflict between the Spirit's power and occult power. Revelation 21:8 lists sorcerers among those excluded from the eternal city.