The central character and narrator, pipe-smoking, 52-year-old Roger Lambert is a professor of divinity, a former Methodist minister who adopted an academic career after the scandalous breakup of his first marriage and his union with Esther, 14 years his junior. He teaches the history of the early Christian heresies at a large university in an unnamed Northeastern city that might be Boston. He is a somewhat dilettantish disciple of Karl Barth, the austere Swiss theologian who fiercely insisted on the utter separateness of the divine and the human, and the utter dependence of the latter on the former. Roger admits to insulating this ''hot Barthian nugget'' in ''layers of worldly cynicism and situation ethics.'' Mr. Updike is well able to evoke the ethos of an academic theology department, and to have sly fun with its professional rivalries, pretensions and jargon (this one has specialists in ''Ethics and Moral Logistics'' and ''holocaustics''). He has manifested an interest in religion and theology in previous books. If there was ever such a species as the Protestant novelist, comparable to that much discussed animal, the Catholic novelist, Mr. Updike may be its last surviving example.