It would be better to further define your question. I allude to: what growth periods are you referring to? The answer to the question could be different based on what period of growth is in question.
Generally, Jesus likened Christianity’s growth to a mustard seed. We all know the parable, and his prophecy was correct based on historical evidence.
Westernization is a geopolitical vocabulary term to describe a system of laws and social structure. Christianity exists in all systems. And, Christianity has and does affect all systems. It has affected all overbearing governments, including this one–even though the original bearers of power tried to prevent Catholicism from creeping into republican society.
Why did the Catholic Monarchial social structures not prevail? It’s my belief that the Monarchs became too caught up in the “world” and national security. They trusted less in God and more in their own schemes. This is the same story we learn of in the Old Testament, so history repeats.
This country’s system of “Big Brother” Federalism is nothing more than a “national brand” of brands. It’s my belief that this erroneous system will continue to spill over until end times. Those of us who are not easily deceived, thanks to God’s grace, see through it. However, the masses will be deceived. The anti-Christ is in the world.