He was born in 2233. Little is known about Cromwell's past: He arrived in his early 20s (he's now 44) and was granted permission to stay due to the blessing and high standing of his successor in the church, who fostered him in the ways of Atom. Eventually, Cromwell proved to be a valuable resource in protecting and aiding the town, and he became an eccentric distraction to most of the citizens of Megaton.
Although he never mentions it, Mother Maya is his wife and female counterpart in his church.
He can be seen standing in the center of town, next to the bomb and praying to it in ankle-deep irradiated water. He even beseeches Atom to "shape his fragile form." He is heard preaching to his audience about the teachings of the Children of Atom, mainly on the subject of "birthing a thousand stars", relating to his (and the Children of Atom's) belief that inside every atom is an entire universe.