I can’t really provide the sources, so I won’t attempt to answer your question per se, but:

1. What other countries do you have in mind besides the US and Germany?

2. What gives you the idea that Catholics largely inhabit the South of the United States? Are you thinking of the Southwest, or southern Louisiana?

The divide primarily has to do with the history of the Reformation and the structure of the Catholic Church. The Catholic Church is currently based in Rome and for the majority of it's existence it has been there. The city itself holds significance for two main reasons. First the city, in Catholic tradition, is where the Apostle Peter was executed by the Romans. Second of all, the Catholic Church, in the fall of the Western Roman Empire wanted to portray itself as the new center of authority. By placing themselves in the ancient capital of the Empire there was political benefits.

As the Church was centered in the South of Europe, control of the Church was in the hands of those more local to it's infrastructure. Though the Church held power throughout Europe and it's priests came from many different nations, the Church's central structure, the Curia, was always staffed and operated by those in proximity to the Church's capital. For reasons such as these, and the fact that the Church held direct political authority over large territories on the Italian Peninsula, Italy was considered by many to be the Church's home.

As I'm sure you know, the Reformation began in Germany. In response to perceived corruptions in the Church's central structure many people became disillusioned with the Church and began to form a religious protest. Before long several German princes had joined this protest. These Protestants spread quickly throughout the North as it was further from the Church's central structure. The Church was like any organization, the closer you are to the center or head, the more you personally benefit from it. The peoples of the South had always been closer and in the eyes of those from the North, the South had always been apart of the problem. And so far from the Church, the Protestants grew immense.

I will not go into the reasons, because it not directly answering your question and is far too complex for me to type on this phone, but the Thirty Years War broke out initially on religious lines. People's such as the Austrians, Bavarians, Italians and Spanish were on the Catholic side. People's such as the Bohemians, Prussians, Swedish and Dutch were on the Protestant side. The war was brutal and only grew worse as more countries entered the war. The sheer violence that had been brought upon Europe would not be forgotten. The combatants, of which I only named a few, identified their opponents as religious enemies but also as personal enemies. The longer the war went on the less the two ideas were differentiated.

The Thirty Years War is only one example of the conflicts that took place between Catholic Peoples and Protestant ones. But the continued conflicts cemented the mutual hatred that they held for each other. It had begun over corruption in the Church, issues that even members of the Church agreed should be addressed. Yet it turned into a centuries long conflict that slaughtered millions. Also in the eyes of each side, they did not identify their enemy as just the religious institution, but also the people that lived in the areas controlled by the institution.

When looking at the US, you can see through the biases very clearly. During the 1800s, there were many Americans that opposed the immigration of Irish, (some) Germans and Italians. Their stated reason was because they were Catholics and they threatened the religious welfare of the American people. Yet many examples of their writings show that they simply considered the immigrants to be different and did not want them around. I'd suggest you look up the Know-Nothing Party of the US for more information.

In conclusion, the religious and geographical divide is present because of the historical circumstances, but it has been cemented by conflicts between the people. In those divide you can see emotions that are comparable to those that you'd see in nationalism. And to a degree, it was this divide that helped push each people seeing themselves as a people and as a nation.

Please let me know if this has been helpful.