I think you (or someone else) posted the church marriage certificate on the family tree on ancestry. That's where I got the St. Agnes from.
The certificate is something that is given to the bride and groom. The church should have a series of record books where baptisms, confirmations, marriages and funerals are recorded. Often, but not always, these books have a little more info on them. It's up to the individual priest.
It's also up to the individual parish as to whether they'll give you a copy of the relevant page or entry. They may give you a copy (I use "give" liberally), they may let you see the entry but not copy it; they may not even let you see it, but will tell you what's on it, as best they can read it. I would try contacting the parish directly, and tell them what kind of information you're trying to obtain (specific birthplace, parents's names, etc).
Do you have John's obituary? It might list surviving brothers or sisters, and you can track him through them.
Also, John may have officially had his name changed. You might be able to track down that document in Cuyahoga County.
Regarding the "Jan Sedlak": There was a John Sedlak in Cleveland at the same time as John Sherlock, so that ship manifest we find might be the wrong guy.
On the 1900 Census, John Sherlock lists his birthplace as Germany, on other documents Austria (including naturalization). There is a slight possibility that he originally had a Hungarian name. There are two names that come immediately to mind: Sulok (which would sound like "Shulock to Americans) and Szulok (which would sound like "Sulock")
Good luck in your search.