This is the parish (parroquia) church of San Miguel Archangel after whom the town is named (the town was founded by a Father Miguel but nothing is actually named for him).
From the outside the church is quite beautiful - the inside, well, there are better. The churches in Atotonilco and Dolores Hidalgo have more interesting interiors - but La Parroquia has an exterior that is truly magnificent.
I understand that the church was designed by a local who had no architectural training but who had studied photographs of European churches - mostly from postcards. If you stand at the base of the church near the front doors and look up it's almost like you're standing in the air near the spires of a big european cathedral, sort of like the architect chopped a larger church in two and threw away the bottom half.
Because La Parroquia is the largest church in town and across from the both the Jardine and Allende's birthplace it is definitely someplace to see and walk through on one of your first days in town. The in-town Biblioteca tour goes through the church quite thoroughly.
I was told, before I got to San Miguel, that November 1st (or 2nd) was the only day you could tour the crypt under the main altar. I totally forgot about going - but after I got home I found written in one of my travel guides (that I never bothered to read while in San Miguel) that you can get into the crypt whenever you want by tipping one of the church workers. Several of the ex-President's of Mexico are in the crypt.