The Reinheitsgebot was enacted primarily for financial/tax and market stability purposes.

The ingredient purity rules were somewhat secondary to other aspects of the law, which were intended to fix prices for grains, establish and standardize distribution and production licenses, and set guidelines to help the tax man and regulators do their job better/easier.

This helped prevent conflicts and grain issues with bakers and others who relied on valuable wheat and rye, and directly regulated prices.

The deployment of the law, like anything at that time, was pretty slow and inconsistent, mostly enacted at the duchy or village level, with some hiccups in northern Germany where they had traditionally produced spiced and fruit beers.

Resistance would have been totally at the individual, family, local, or at most the guild level...as grain markets were complex, beer taxes were greatly relied on, and it was a monarchy...so very little serious or organized resistance overall. It would be sensible to assume families or small brewers occasionally spiced up their beer or had house recipes that snuck in rye or some other ingredients, made for special occasions or private consumption under the counter. I have no good proof of this, but brewers are a creative and rebellious bunch. The law was essentially valid and unchanged in spirit up through the 1990s (and continues voluntarily, or within local laws such as munich's oktoberfest criteria).

As the law spread and was enforced many styles became defined, and eventually the simple pilsner became king, somewhat out of accident, but soon perfected.

source-Dornbusch, Horst D. (1997). Prost!: The Story of German Beer.

and also here's an EN translation of the 1516 law http://brewery.org/library/ReinHeit.html