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Upon completing his studies in 1966, William was [[ordination|ordained]] to the diaconate on [[July 17]] and to the [[Presbyter|priesthood]] in the Romanian [[Uniate]] Church on [[October 23]]. Returning to the United States in January 1967, he was appointed as the assistant priest in the [[parish]] of St. Michael, in Aurora, Illinois. As Fr. Popp entered into parish life, the spirituality and theology of his Orthodox experiences began to rise in importance for him. Within months he sought entrance into the Orthodox Church. Having no practical experience in American Orthodox situation, he visited former classmates from Rome who had also embraced Orthodoxy. Through the parish priest of St. Mary parish in Cleveland, Ohio, Fr. Popp was introduced to Archbishop [[Valerian (Trifa) of Detroit|Valerian (Trifa)]] of the Romanian Orthodox Episcopate of America,

Under the guidance of Archbishop Valerian, Fr. Popp was examined by the Episcopate Council and received into the Orthodox faith at St. Mary [[Chapel]] at ‘’Vatra’’ on [[February 15]], 1968. After residing in a [[Monasticism|monastic]] community for several years, Fr. Popp became the priest of Holy Cross Parish in Hermitage, Pennsylvania in 1975. On [[September 20]], 1980, a Special Electoral Congress of the Romanian Orthodox Episcopate of America called Fr. Nathaniel to the episcopate as an [[auxiliary bishop]] to Abp. Valerian.