During this time, he was tonsured a monk and received the small schema as a fifth-year student. The tonsuring was done by Bishop Stefan of Dalmatia on the eve of the school Slava of the Holy Three Hierarchs on February 11, 1975. At the Divine Liturgy on February 12th, he was ordained a deacon by Bishop Stefan. On February 13th, he was ordained a priest.

He entered the Moscow Theological Academy in 1975, where he graduated in 1979. From October 1980 to April 1981, he served in the Diocese of Zvornik-Tuzla as secretary of the Executive Board. He was also administrator of two parishes. On the decision of the Holy Synod of Bishops, he was appointed as lecturer of the Holy Three Hierarchs Seminary in 1983 for two years.

At the Sabor of May 1985, he was elected Vicar Bishop of Morava to the Patriarch. He was consecrated on October 20, 1985. His Holiness served the Divine Liturgy along with Metropolitan Vladimir of Rostov and Novo Cerkay (now Metropolitan of Kiev) who had been Dean of the Moscow Theological Academy while Bishop Longin was a student. Also serving were Bishop Nikolaj of Dalmatia, who had been Dean at the Seminary in Krka Monastery where the young Bishop had begun his theological education, and Bishop Vasilije of Zvornik-Tuzla.

Present at the services were the following bishops of the Serbian Church: Metropolitan Vladislav of Dabro-Bosna, Metropolitan Jovan of Zagreb and Ljubljana, Bishop Chrysostom of Branicevo, Bishop Pavle of Ras and Prizren, Bishop Stefan of Zica, and Bishop Jefrem of Banja Luka. As guests present were the Archimandrite Platon, professor of the Moscow Theological Academy, Canon Dr. Colin Hickling, professor of Holy Scriptures at the London University, where the new bishop had done postgraduate studies. In May of 1986, he was elected Bishop of Australia and New Zealand, where he spent six and a half years. Besides serving the great territory of Australia, he also built churches and was involved in work with the youth. Bishop Longin was known especially as a fighter for the unity of the Serbian Church.

At the Sabor in 1992, at the request of His Holiness and brother hierarchs, Bishop Longin accepted the duty of being bishop of Dalmatia. He was not able to enter his residence in Sibenik, but had to reside in Monastery Krka, where he lectured at the seminary. With the blessing of His Holiness Patriarch Pavle, he journeyed to Canada and Australia, where he collected donations for the suffering Serbian people. After the tragic occurrences in August of 1995, Bishop Longin and the entire seminary from Monastery Krka stayed at Divcibare near Valjevo. At the same time, he helped Bishop Lavrentije of Sabac-Valjevo in diocesan matters. Along with Bishop Nikanor of Gornji Karlovac, he worked with refugees, visited the camps, and delivered help which had been sent mainly by the Serbs in the Diaspora.

With the decision of His Holiness Patriarch Pavle, he was appointed on July 31, 1997, as auxiliary in the administration of the Diocese of America and Canada – New Gracanica Metropolitanate. On May 23, 1998, he was appointed as Administrator of the Diocese of America and Canada – New Gracanica Metropolitanate, while on May 5, 1999, he was elected at the Holy Assembly of Bishops of the Serbian Orthodox Church, Sabor, as Bishop of the Diocese of America and Canada, New Gracanica Metropolitanate. This fulfilled the request of the Diocesan Council and the Ecclesiastical Court as well as the desire of the clergy and delegates of the Seventh Church National Sabor held on April 23 – 24, 1999.

After the administrative arrondation of our Church on the North American Continent in May 2009, a better and canonically more correct organization of our faithful people was achieved. In the same year, Bishop Longin was elected Bishop of New Gracanica-Midwestern America. Since then, His Eminence has resided in Libertyville, retaining offices at Pokrov-Protection Monastery. The intention is for our oldest Monastery to be renovated in many ways, so that all monasteries and parish churches can successfully continue the mission of salvation and deliverance from all evil! For this, we pray to the Lord!

During the Holy Assembly of Bishops (May 14 – 18, 2024) of the Serbian Orthodox Church in Belgrade, the Sabor decided that the title of the diocesan hierarch of the Diocese of New Gracanica-Midwestern America, headquartered at the Monastery of St. Sava in Libertyville or at the Monastery of the Protection of the Most Holy Mother of God (New Gracanica) at Third Lake, henceforth reads: His Eminence Archbishop of New Gracanica – Chicago and Metropolitan of Midwestern America.