This post will focus on two of the most important churches in Istanbul, neither of which are active churches. The one of course is Hagia Sophia, the other St. Irene (i.e. Eirene). 
Hagia Sophia was originally built in 360 A.D. by Emperor Constantius, the son of the famous Constantine. Almost nothing remains of this original church (but see below) and so the building you now see was built between 532 and 537 A.D. It did not become a mosque until 1453 when the Osmanlis (Ottomans) had conquered the city and ruled the land of Turkiye. The Christian Emperor Justinian 1 built the church you can now see.
Below is about all that is left of the original church…
Notice the sheep and the palm trees.
In the main entrance way to the church you can still see this mosaic of Justinian on the left offering Mary and Jesus the church, whereas on the right you have Constantine offering them the city (and the Empire presumably).
The now central entranceway lacks religious symbols.
Certainly this building with its lack of columns supporting the vast span of central dome was one of the engineering marvels of its age. Note the gray dome below.
And now from the inside….
Notice the six winged seraphim.
Some of the Christian symbolism has been covered up with Arabic signs, quotes from the Koran presumably, or their creed, there is one God Allah, and Mohammed is his prophet.
Despite their best efforts, some of the Christian symbolism are still visible with the help of a zoom lens…
In this image we have Mary, Christ, the archangel Gabriel and the Emperor doing obeisance to Christ.
And see also in the distance in the now closed upper gallery….
On the side of the building, one can see original decorated doors…
Yes there are now regular Muslim services in here, but Christian ones are not allowed. This is a travesty, for long before this was a mosque, this was the first great and central church of Christianity, just as Constantinople for a time was the main holy city of Christendom.
Since the beautiful Chora Church on the edge of the old city is now closed, we visited the historically important St. Irene’s where the council of Constantinople was held, and possibly the Council of Chalcedon.
This church has not been turned into a mosque (yet) and is on the grounds of Topkapi Palace at the summit of that portion of the city. You get to it through the main gate into Topkapi.
The outside looks a bit like Hagia Sophia.
The inside is bare and undecorated, but you can see a council meeting here.
A baptismal font. It was one of the first churches to be built in the new Christian capital city in 330 A.D. and was built on the site of an old pagan temple.
As for a purpose built genuine mosque there is the famous Blue mosque (so called because of the beautiful blue tiles inside it) across from the Hagia Sophia now under major renovation, but there is still a little to see. Here is the outside under repair…
It is famous for its six minarets or prayer towers…
Here’s an inside shot which gives you a sense of the mosque’s color,
And here is a prayer place….
And a pulpit followed by stained glass windows. The Muslim tradition is anti-iconic, by which I mean no images of God or humans in the mosque…
Yes from time to time a sermon would be given by a cleric. 
Just across the way, there is a study center for Koran students, and you can also see their creed posted for all to read in English.
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