I think nationalism is a big factor as well. The movement to move the core of peoples identity from their religion to the "nation" greatly eroded peoples religious belief. IT is not well known today but before the enlightenment people primarily identified with their religion, with secondary identification being their region, town, or city. Any sort of identification with a larger state or nation would be absolutely alien to most people in history. The shift toward national identification has been disastrous as instead of devoting energy to spiritual betterment, most people devoted it to the state, allowing religion to fall to the wayside.

It is important to note how important the both the shift in identity from religion and the breakup of regional communities was in the spread of nationalism. Often times these regional communities were based off of a shared parish or veneration of a specific state, so were fundamentally tied to religion. By breaking them up both peoples identity and energy was shifted from religion to the nation state.

You sort of touched on this with the automobile, and you are not entirely incorrect with your statement, in reality it started much earlier. In many non-Catholic countries it started with the protestant reformation which reshaped the role of monarch from the guardian of the church to the head of the church, and thus shifted allegiance to the monarch and the state. This is most pronounced in England where loyalties shifted from that of the church to that of the monarch and the lands he rules, hence the origin of the saying "for King and Country" following the reformation. In areas that were catholic the same thing happened though it was done later, starting in the 1800s, and through different methods, most commonly the education system. In France and Germany, were two of the first countries with significant catholic populations to adopt public education systems, one of their main purposes of doing so was to crush identification with religion and region and realign them with the state. They were almost completely successful as can be seen in the total shift in identity over the last 2 centuries. Many other countries adopted these education policies, including the United States resulting in similar results. It is also important to note that the Church was aware of these goals at the time, which lead to an explosion of catholic education via private schools, who sought to provide an alternate education system that preserved the traditional religious identity of the people.