Complex sentence: Besides the Patriarchate-affiliated church, Latvia has a number of priestless congregation of the Grebenstchikov House of Prayer in Riga, affiliated with the Pomorian Old-Orthodox Church, is considered the oldest extant Old Believer congregation in the world.
Split into simpler sentences: Besides the Patriarchate-affiliated church, Latvia has a number of Old Believer Orthodox Christian communities as well. The priestless congregation of the Grebenstchikov House of Prayer in Riga, affiliated with the Pomorian Old-Orthodox Church, is considered the oldest extant Old Believer congregation in the world.