/ Instrumental form in Polish
All right - we know what cases the nouns should take, so let's get to the word order(WO). You can roughly say that WO in a Polish sentence is free, but it is only an approximation. Some very weird WOs just aren't used ("Poszedł do wczoraj biblioteki Wojtek") and there is nothing complicated about it. Sometimes we change word order for emphasis, which isn't very difficult either.
The problem of the WO begins if we consider Polish sentences within a context.
Out of context you can say:Zobaczyłam mężczyznę na ulicy.
Na ulicy zobaczyłam mężczyznę.
Mężczyznę zobaczyłam na ulicy.
(I saw a/the man in the street.)
It looks as if it meant the same but if we put it into a text, we'll see that WO matters. *
" Wyszłam wczoraj rano z domu.
(I went outside yesterday morning) Na ulicy zobaczyłam mężczyznę.
" is OK, while "Wyszłam wczoraj rano z domu. Mężczyznę zobaczyłam na ulicy.
"is not. But:
" Zauważyłam w Warszawie pewne młode małżeństwo.
(I spotted a young couple in Warsaw) Mężczyznę zobaczyłam na ulicy. Jego żonę zobaczyłam w sklepie obok.
(I saw his wife in a shop nearby)"
is OK but it wouldn't be if we used the WO that was correct for Ex. 1.
In this case the problem is not so difficult. Let's try to translate those two short texts into English as a whole:
I went outside yesterday morning. I saw a man in the street.
I spotted a young couple in Warsaw. I saw the man in the street.
I saw his wife in a shop nearby.
In English translation it is easy to see, that 'man' in ex. 1 is indefinite noun, and in ex. 2 is definite. We can see it thanks to articles.
In Polish there are no articles but there are still definite and indefinite nouns (noun phrases). And if definiteness is indicated by any syntactic feature, it is indicated by the WO. If some noun is definite, it is a very particular one, it will usually go to the beginning of the sentence. An extreme example:
Na stole jest piwo. - There's a beer on the table.
Piwo jest na stole. - The beer is on the table.
Quite a big change of meaning.
But it doesn't mean that you can always guess definiteness from the WO. Sometimes it's impossible to say if something is or isn't definite.
But in this goddmamn sentence "Największą przeszkodą był Kościół", there are two definite nouns so why the hell can't it be "Kościół był największą przeszkodą"? The only explanation that came to my head is based on the following interpretation of what a sentence is. It's a bit controversial idea but it works for this problem, so let's accept it.
Each sentence is composed of two main parts. You always take some well-known thing and provide some new information about it. When you say "Jack is a plumber" you assume that your listener knows who Jack is and the new information you convey is his profession. So, to use those fancy linguistic words, Jack is the theme / topic
of the sentence and the fact that he's a plumber is the rheme / focus /comment
. SO, that's basicly all. Some more examples:
1. Jordan ate a green apple - th: Jordan; rh: ate a green apple
2. The green apple was eaten by Jordan - th: The green apple; rh: was eaten by Jordan
Kate likes raw beef - th: Kate; rh: likes raw beef
There is a big album on the shelf: - th: on the shelf; there is a big album
What I told him was that he was a jerk: - th: What I told him; rh: was that he was a jerk.
Of course, in a sentence where there is one definite and one indefinite noun phrase, it's natural that the definite one will be the theme. "The A" is better-known than "a B". The idea of definiteness and the idea of a theme are in some sense similiar.
If you look again at the examples, you will probably notice that the theme tends to be placed in the beginning of the sentence. The only exception I gave is an existential sentence (a 'there is/there are' sentence). In the pair about Jordan and the green apple you can see it very clearly: the passive voice in the second sentence ("The apple was eaten by Jordan") is a form of emphasis. It could be "Jordan ate the apple", but in passive voice the apple gets to the beginning of the sentence and thus, emphasizes the fact that we're talking about the apple, not Jordan. The theme-rheme difference becomes stronger by moving the 'apple' to the front.
In Polish it's the same: the best place for the theme is in the beginning. But in Polish, if I wanted to emphasize that the apple is the theme of the sentence, I'd move it to the front just like that. I really don't need to change the sentence into passive voice: "Jabłko zjadł Jordan" is enough.
So let's approach the church-obstacle sentence in a similar way.
You wrote something about fighting for women's rights. If there was any fighting, we know that there was an obstacle. It's given. Now, you want to identify it - so you want to give the 'new info' about the obstacle. So: the obstacle is the theme of your sentence, the rest is the rheme and the 'biggest obstacle' goes to the front.
Outside of context (so when it's sometimes hard to say which is which), the most natural word order in Polish seems to be the one that has the subject (which is always nominative) in the beginning. But your sentence is a part of a text, so the word order should go according to what this sentence means within this text. Even if it means putting the grammatical subject in the end.
"Kościół był największą przeszkodą" is grammatically correct and it would fit a text where something more is said about the church before. E.g:
They hoped that the church would support them. In the beginning they even believed it but in fact, it was just the contrary. The church was the biggest obstacle.
In English 'the church' goes to the beginning of the sentence (because it's definitely the theme here). Doing this, it becomes the subject of the sentence because that is how English grammar works. In Polish it's also at the beginning "Kościół był największą przeszkodą" but in Polish all the cases and syntactic relationships are the same as in "Największą przeszkodą był kościół"- kościół is the subject, największa przeszkoda is the predicate. The cases are ruled by those things I described in part 1, and they're independent from the WO thing.
*(The analysis is based on the assumption that the context I am giving is the only context known, so please, don't provide any additional context as counter-arguments)