Archbishop Nathaniel of Detroit, born William George Popp on 12 June 1940 in Aurora, Illinois, to a Romanian-American family from Satu Mare County, serves as the ruling hierarch of the Romanian Orthodox Episcopate of America within the Orthodox Church in America.

Raised attending Saint Michael the Archangel Romanian Greek Catholic Church, he pursued priestly formation at Saint Procopius Seminary in Lisle, Illinois, and the Pontifical Gregorian University in Rome, where he was ordained to the priesthood on 25 March 1966 in the Romanian Greek Catholic Church.

Experiencing a deepening attraction to Orthodox theology during his studies abroad, Nathaniel Popp returned to the United States and, under the guidance of Archbishop Valerian Trifa, was received into Orthodoxy on 15 February 1968 at the Vatra Românească chapel in Grass Lake, Michigan.

Following his conversion, Father Nathaniel resided in a monastic community at the Vatra for several years before serving as assistant priest at Saint Michael Church in Aurora and then as rector of Holy Cross Romanian Orthodox Church in Hermitage, Pennsylvania, beginning in 1975.

On 20 September 1980, a special congress of the Romanian Orthodox Episcopate elected him to the episcopate, leading to his consecration as Bishop of Dearborn Heights, an auxiliary to Archbishop Valerian, on 15 November 1980.

Upon Valerian's retirement in 1984, Bishop Nathaniel succeeded as the locum tenens and was formally elected ruling bishop of the episcopate on 17 November 1984, a position he has held continuously.

The Holy Synod of the Orthodox Church in America elevated him to archbishop on 20 October 1999, recognizing his leadership in fostering growth, including the establishment of 39 new missions and parishes, two monastic communities, and initiatives like the Help for Romania Fund and Help the Children of Romania.

Archbishop Nathaniel has chaired the V. D. Trifa Romanian American Heritage Center, served as president of the Congress of Romanian Americans since 1991, and acted as episcopal moderator for pastoral life ministries in the Orthodox Church in America, while advocating for Orthodox unity in North America.

Residing in Detroit, he marked 40 years of hierarchical service in 2020, continuing to guide the episcopate's preservation of Romanian Orthodox traditions amid American societal integration.