Simplify the following text for a Grade 16 native English-speaking person.
Original: Establishment of the Diocese and the Archdiocese and the cities of Bialystok ended the period of the temporary church administration in the Bialystok region owned lands over the centuries to the Roman Catholic Archdiocese of Vilnius , which after World War II remained in the Polish borders .
Simplified: Pope John Paul II on 5 June 1991 , during a visit to Bialystok , announced the establishment of the Archdiocese of BiaÅ ystok which ended the period of the temporary church administration of the portion of the Archdiocese of Vilnius that had , after World War II , remained within the Polish borders .