This paper focuses on John Locke’s interest in pre-existence and transmigration, which emerges in his greatest work An Essay concerning Human Understanding and elsewhere in his writings. It aims to show that this interest was stimulated by his reading of Christian Knorr von Rosenroth’s Kabbala denudata, which gave a prominent position to those theories, and the writings of the Flemish Kabbalist Francis Mercury van Helmont, a strenuous advocate of metempsychosis who collaborated with Knorr on publishing Kabbalistic texts. I argue that Locke was intrigued by their opinions for two important reasons. First of all, both Knorr and van Helmont leveraged these theories to undermine the Christian doctrines of original sin and the eternity of hell, which Locke likewise rejected. Secondly, the controversy stirred up by van Helmont’s opinions brought to the fore the question of what might ensure the preservation of personal identity over time, an issue that was much discussed in the seventeenth century in connection with the two related questions of the immortality of the soul and the resurrection of the body. Locke was particularly interested in these debates, as is shown by the Essay.