Hardenbergh studied theology under the Reverend John Frelinghuysen, a Reformed clergyman who served several congregations located in the valley of the Raritan River in Somerset County, New Jersey, at the village of Raritan, Bedminster, Millstone, Neshanic, North Branch, and New Brunswick. These congregations had been served by Frelinghuysen's father, the Reverend Theodorus Jacobus Frelinghuysen (1691–1749), who rose to prominence during the First Great Awakening. The First Great Awakening was an evangelical revival in the 1730s and 1740s that revitalized Christian beliefs in Protestant Europe and the North American colonies through powerful preaching that gave listeners a sense of deep personal revelation of their need of salvation by Jesus Christ. Frelinghuysen had adapted the theological developments of the Puritan divines to preach a style of Reformed pietism, a revivalistic style of Calvinism. His son, John, preached and instructed his students in the same style.