‘Experimental Heresies’ examines how psychiatry and Christianity constructed the value of LSD and other psychedelic substances in mid-twentieth century Britain, and how this informed wider cultural, social, and legal readings. Using the philosopher Williams James’ notion of primary and secondary religious experience, this chapter begins by dealing with the LSD research of psychiatrist Dr Frank Lake in the 1950s and early 1960s. Lake investigated LSD as an abreactive agent, which he explored as part of his development of a form of pastoral care called Clinical Theology. His work, a Christian and psychiatric collaboration, explicitly maintained conservative social boundaries around sexual behaviour and the sanctity of family in patients. While these particular boundaries were liberalised over the same period, the use of psychedelic substances was legislated against as non-clinical use proliferated during the 1960s. Following Thomas Szasz’s idea of the ‘pharmacracy’, this chapter then examines how the value of LSD began to shift as it was increasingly used and excluded through its countercultural associations. The importation of a religious and mystical discourse around LSD from the United States posed a peculiar problem for British Christian pastoral carers, especially against the backdrop of a severe decline in Church attendance by the young. By looking at the writings of Curate, Kenneth Leech, this chapter concludes by describing the effect of the ‘LSD heresy’ on how some members of the Church understood its own failure to administer spiritually to the young.