Complex sentence: The Catholic population, mostly of Polish origin, was constantly increasing, and permission was given to build a church in 1855, on condition that the church does not stand out from the surrounding buildings and not having a strong Catholic architectural character.
Split into simpler sentences: The Catholic population, mostly of Polish origin, increased constantly and permission was considered to build the church in 1855. The permission, for the construction of the church, was granted with some conditions such as; the church does not stand out from the surrounding buildings and not having a strong traditional Roman Catholic architectural character.