Istanbul is not all about mosques and Islam. There are also a few pretty nice Christian churches hidden within the city. This is a pretty good indicator of city's cultural diversity.
One of them is definitely the Church of St. Anthony of Padua (aka Sant'Antonio di Padova). It is situated just off the İstiklal Avenue in the Beyoğlu district.
It is quite nice and worth a quick look if you are in the Beyoğlu neighbourhood, but not so special to deserve a dedicated visit from the opposite side of the city.
It was built in 1725 by members of Italian community, but later demolished and rebuilt again between 1906 and 1912. It is actually considered a minor basilica and is run by Italian priests.
According to Wikipedia Pope John XXIII preached in this church for a period of 10 years (when he was the Vatican's ambassador to Turkey) just before being elected as pope. He is known in Turkey with the nickname "The Turkish Pope" because of his fluent Turkish and his often expressed love for Turkey and the city of Istanbul.