Good reference ~ someone with some knowledge of the Restoration belief made sure to show the connection (via dotted line) back to the time of the Christian congregation in Jerusalem.
Was at a funeral last month, in one of the yellow counties on that map. Turned out to be a Church of Christ. The dedication stone said "Founded by Jesus Christ in AD 33". It was all very happy-clappy evangelical, except for one passing comment by the presiding minister that, if you were paying attention, pointed to their belief in necessity of baptism for salvation. It didn't seem the time or place to raise a ruckus about it.
I'm not sure how useful the map is. According to it, we should be tripping over Roman Catholics here. They are there, but so's most everybody else. Driving down the street, you're far more likely to see a mainline Protestant church or one of these generic non- (or weakly) denominational evangelical churches.
I was somewhat intrigued with this map ever since I did an intense survey of the Rand McNally religious survey map 30 years ago. At that time Indiana was the only state that reported having no counties with a majority composed of members of a single denomination.
Now, it has such a county, and some of those earlier reported as highly mixed are now shown to be 50% or more Catholic.
I suspect what we have here is useless for planning purposes! Or, the Catholics have been making striking gains.