In the 1864 St. Louis,Missouri City Directory (viewed electronically here on Ancestry) I find the following entry on Page 275 of 487:
McLaughlin John, lab., Market, se c. Beaumont
This is the only entry for a John McLaughlin in that directory. I looked back at your old post and saw that your John McLaughlin was a carpenter, but sometimes was also shown as a laborer.
The address given in the directory is the corner of Market Street and Beaumont. This is just west of Jefferson Avenue, and today would be part of the campus of Wells Fargo Advisors, on the western end of Downtown near midtown.
Today, the closest parish church would probably be St. Francis Xavier (College) church on the campus of Saint Louis University. However, a look at their website (see link below) indicates that they did not move to this location until 1867.http://www.slu.edu/college-church/who-we-are/early-history
St. John the Evangelist Church is also not too far away. It looks like that church has been at their present location since 1850:http://www.stjohnapostleandevangelist.org/history.html
I'm not entirely sure what parish(es) would have served this area in the early 1860's, but I bet the folks at the archives would know and would be able to check the right records for you.