There are cities in the world that accept immigrants because of various reasons. For example the city I lived in, Mashhad, absorbs immigrants because of religious reasons. People like to be there because they want to be near their important religious person tomb. So what is this kind of city called?, A city that many people liked migrating to and migrated to it through the history.
It appears that scholarly literature uses the phrase immigrant gateways.
The Brookings Institution article The Rise of New Immigrant Gateways defines six types of immigrant gateway:
Historical settlement patterns along with recent influxes of immigrants have produced six major types of U.S. immigrant "gateways." Former gateways, like Cleveland and Buffalo, attracted immigrants in the early 1900s but no longer do. Continuous gateways such as New York and Chicago are long-established destinations for immigrants and continue to receive large numbers of the foreign-born. Post-World War II gateways like Los Angeles and Miami began attracting immigrants on a grand scale during the past 50 years. Atlanta, Dallas, and Washington, D.C., meanwhile, stand out as emerging gateways with fast immigrant growth during the past 20 years. Seattle and the Twin Cities—places that began the 20th century with strong immigrant pulls—waned as destinations during the middle of the century, but are now re-emerging as important immigrant gateways. Finally, Salt Lake City and Raleigh-Durham are very recent immigrant destinations, having attracted significant numbers of immigrants in the 1990s alone. These are the pre-emerging gateways.
'Gateway' may have currency in the US, but it certainly doesn't resonate as an appropriate descriptor in Australia, nor as far as I'm aware in the wider Anglosphere. In the English-speaking world, we are of course familiar with large-scale economic migration and would commonly use 'Migrant destination' to describe such a city.
If it's for religious reasons as you state - or even for sporting, cultural events, climate or lifestyle issues - then the most idiomatic term that I'm aware of is 'a mecca' with a modifier to describe the source of attraction eg. "Hobart has become a mecca for environmentalists". Note the use of the indefinite article 'a' - if you were talking about the original Mecca in this context then you would talk about 'THE Mecca'.