[1][2] Born to a Romanian-American family in Aurora, Illinois, he was ordained to the priesthood in 1966, in the Romanian Greek-Catholic Church.

He soon left the Catholic Eastern Rite and, under the guidance of Archbishop Valerian Trifa of the Romanian Orthodox Episcopate of America, converted to Orthodoxy on February 15, 1968.

Popp became the rector of Holy Cross Romanian Orthodox Church in Hermitage, Pennsylvania.

On November 17, 1984, Bishop Nathaniel became the ruling hierarch of the Romanian Orthodox Episcopate of America; on October 20, 1999, the Holy Synod of the OCA elevated him to the rank of Archbishop.

Following the resignation of Metropolitan Jonah (Paffhausen) in July 2012, Bishop Nathaniel was appointed as the locum tenens of the OCA.