Theuner was ordained to the diaconate in June 1962 and to the priesthood in December 1962 by the Bishop of Ohio Nelson M. Burroughs.

He was consecrated on April 19, 1986, in St Joseph Roman Catholic Cathedral in Manchester, New Hampshire.

As bishop he was very supportive of the inclusion of homosexual people, and was a fierce supporter of the election and consecration of his successor Gene Robinson to be the first openly gay bishop in the Episcopal Church.

Theuner married Jane Lois Szuhany and together had two children.

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