Becoming fluent in French early on, he later studied theology at Yale, where he earned his degree in 1752, followed by his preacher's license in 1756.

He then served as an itinerant preacher to the Dutch and Huguenot parishes in Jamaica and Long Island, New York, where he gained much popular support.

By 1776, Keteltas was elected to the Provincial Congress and became such a vociferous defender of the American cause that he feared for reprisals when British troops landed on Long Island.

During the American Revolution, he served as preacher to a number of Presbyterian churches in Massachusetts and Connecticut until his retirement in 1782.

The Religious Soldier (1759), preached to American and British forces in 1759, exhorts his audience to moral conduct in warfare and patriotic service of their country.